Merge bitcoin-core/secp256k1#1382: refactor: Drop unused cast

4f8c5bd761 refactor: Drop unused cast (Hennadii Stepanov)

Pull request description:

ACKs for top commit:
  real-or-random:
    utACK 4f8c5bd761
  jonasnick:
    ACK 4f8c5bd761

Tree-SHA512: cc94b524f53e393bd843383e92bbc5b84dd7557d8121241f2d0461b960a0706236147d02b6f5bfc433272849f517c62eb6f1e0cfae892e1b8054817c27365430

src/hash_impl.h

@@ -138,7 +138,7 @@ static void secp256k1_sha256_write(secp256k1_sha256 *hash, const unsigned char *
     }
     if (len) {
         /* Fill the buffer with what remains. */
-        memcpy(((unsigned char*)hash->buf) + bufsize, data, len);
+        memcpy(hash->buf + bufsize, data, len);
     }
 }