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

src/ecdsa_impl.h

@@ -140,7 +140,7 @@ static int secp256k1_der_parse_integer(secp256k1_scalar *r, const unsigned char
         overflow = 1;
     }
     if (!overflow) {
-        memcpy(ra + 32 - rlen, *sig, rlen);
+        if (rlen) memcpy(ra + 32 - rlen, *sig, rlen);
         secp256k1_scalar_set_b32(r, ra, &overflow);
     }
     if (overflow) {