refactor: Use array initialization for unterminated strings

The previous code is correct and harmless to initialize an array with a
non-terminated character sequence using a string literal.

However, it requires exactly specifying the array size, which can be
cumbersome.

Also, GCC-15 may issue the -Wunterminated-string-initialization warning.
[1]

Fix both issues by using array initialization. This refactoring commit
does not change behavior.

[1] Example warning:

src/modules/schnorrsig/main_impl.h:48:46: error: initializer-string for array of 'unsigned char' is too long [-Werror=unterminated-string-initialization]
   48 | static const unsigned char bip340_algo[13] = "BIP0340/nonce";
      |                                              ^~~~~~~~~~~~~~~

examples/schnorr.c

@@ -18,9 +18,9 @@
 #include "examples_util.h"
 
 int main(void) {
-    unsigned char msg[12] = "Hello World!";
+    unsigned char msg[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!'};
     unsigned char msg_hash[32];
-    unsigned char tag[17] = "my_fancy_protocol";
+    unsigned char tag[] = {'m', 'y', '_', 'f', 'a', 'n', 'c', 'y', '_', 'p', 'r', 'o', 't', 'o', 'c', 'o', 'l'};
     unsigned char seckey[32];
     unsigned char randomize[32];
     unsigned char auxiliary_rand[32];

src/modules/ellswift/tests_impl.h

@@ -406,9 +406,9 @@ void run_ellswift_tests(void) {
     /* Test hash initializers. */
     {
         secp256k1_sha256 sha, sha_optimized;
-        static const unsigned char encode_tag[25] = "secp256k1_ellswift_encode";
-        static const unsigned char create_tag[25] = "secp256k1_ellswift_create";
-        static const unsigned char bip324_tag[26] = "bip324_ellswift_xonly_ecdh";
+        static const unsigned char encode_tag[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', '_', 'e', 'l', 'l', 's', 'w', 'i', 'f', 't', '_', 'e', 'n', 'c', 'o', 'd', 'e'};
+        static const unsigned char create_tag[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', '_', 'e', 'l', 'l', 's', 'w', 'i', 'f', 't', '_', 'c', 'r', 'e', 'a', 't', 'e'};
+        static const unsigned char bip324_tag[] = {'b', 'i', 'p', '3', '2', '4', '_', 'e', 'l', 'l', 's', 'w', 'i', 'f', 't', '_', 'x', 'o', 'n', 'l', 'y', '_', 'e', 'c', 'd', 'h'};
 
         /* Check that hash initialized by
          * secp256k1_ellswift_sha256_init_encode has the expected

src/modules/schnorrsig/main_impl.h

@@ -45,7 +45,7 @@ static void secp256k1_nonce_function_bip340_sha256_tagged_aux(secp256k1_sha256 *
 
 /* algo argument for nonce_function_bip340 to derive the nonce exactly as stated in BIP-340
  * by using the correct tagged hash function. */
-static const unsigned char bip340_algo[13] = "BIP0340/nonce";
+static const unsigned char bip340_algo[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'n', 'o', 'n', 'c', 'e'};
 
 static const unsigned char schnorrsig_extraparams_magic[4] = SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC;
 

src/modules/schnorrsig/tests_impl.h

@@ -21,9 +21,9 @@ static void nonce_function_bip340_bitflip(unsigned char **args, size_t n_flip, s
 }
 
 static void run_nonce_function_bip340_tests(void) {
-    unsigned char tag[13] = "BIP0340/nonce";
-    unsigned char aux_tag[11] = "BIP0340/aux";
-    unsigned char algo[13] = "BIP0340/nonce";
+    unsigned char tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'n', 'o', 'n', 'c', 'e'};
+    unsigned char aux_tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'a', 'u', 'x'};
+    unsigned char algo[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'n', 'o', 'n', 'c', 'e'};
     size_t algolen = sizeof(algo);
     secp256k1_sha256 sha;
     secp256k1_sha256 sha_optimized;
@@ -158,7 +158,7 @@ static void test_schnorrsig_api(void) {
 /* Checks that hash initialized by secp256k1_schnorrsig_sha256_tagged has the
  * expected state. */
 static void test_schnorrsig_sha256_tagged(void) {
-    unsigned char tag[17] = "BIP0340/challenge";
+    unsigned char tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'c', 'h', 'a', 'l', 'l', 'e', 'n', 'g', 'e'};
     secp256k1_sha256 sha;
     secp256k1_sha256 sha_optimized;
 
@@ -806,7 +806,7 @@ static void test_schnorrsig_sign(void) {
     unsigned char sk[32];
     secp256k1_xonly_pubkey pk;
     secp256k1_keypair keypair;
-    const unsigned char msg[32] = "this is a msg for a schnorrsig..";
+    const unsigned char msg[] = {'t', 'h', 'i', 's', ' ', 'i', 's', ' ', 'a', ' ', 'm', 's', 'g', ' ', 'f', 'o', 'r', ' ', 'a', ' ', 's', 'c', 'h', 'n', 'o', 'r', 'r', 's', 'i', 'g', '.', '.'};
     unsigned char sig[64];
     unsigned char sig2[64];
     unsigned char zeros64[64] = { 0 };

src/testrand_impl.h

@@ -18,7 +18,7 @@
 static uint64_t secp256k1_test_state[4];
 
 SECP256K1_INLINE static void testrand_seed(const unsigned char *seed16) {
-    static const unsigned char PREFIX[19] = "secp256k1 test init";
+    static const unsigned char PREFIX[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', ' ', 't', 'e', 's', 't', ' ', 'i', 'n', 'i', 't'};
     unsigned char out32[32];
     secp256k1_sha256 hash;
     int i;