Merge #879: Avoid passing out-of-bound pointers to 0-size memcpy

9570f674cc Avoid passing out-of-bound pointers to 0-size memcpy (Pieter Wuille)

Pull request description:

  Doing so could be considered UB in a pedantic interpretation of the standard. Avoid it.

  Closes #876.

ACKs for top commit:
  practicalswift:
    cr ACK 9570f674cc: patch looks correct
  real-or-random:
    ACK 9570f674cc

Tree-SHA512: f991462d72e39f14e609021b8427c2e6756009bc8cd21efca2da46ec9410250725a4fed662df20fcdcfd10a4dc59038f13e8c166362b2eadde4366586b9ca72b

contrib/lax_der_parsing.c

@@ -120,7 +120,7 @@ int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_
     /* Copy R value */
     if (rlen > 32) {
         overflow = 1;
-    } else {
+    } else if (rlen) {
         memcpy(tmpsig + 32 - rlen, input + rpos, rlen);
     }
 
@@ -132,7 +132,7 @@ int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_
     /* Copy S value */
     if (slen > 32) {
         overflow = 1;
-    } else {
+    } else if (slen) {
         memcpy(tmpsig + 64 - slen, input + spos, slen);
     }