Use the explicit NULL macro for pointer comparisons.

This makes it more clear that a null check is intended. Avoiding the
 use of a pointer as a test condition alse increases the type-safety
 of the comparisons.

(This is also MISRA C 2012 rules 14.4 and 11.9)

src/ecmult_gen_impl.h

@@ -159,7 +159,7 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
     secp256k1_rfc6979_hmac_sha256_t rng;
     int retry;
     unsigned char keydata[64] = {0};
-    if (!seed32) {
+    if (seed32 == NULL) {
         /* When seed is NULL, reset the initial point and blinding value. */
         secp256k1_gej_set_ge(&ctx->initial, &secp256k1_ge_const_g);
         secp256k1_gej_neg(&ctx->initial, &ctx->initial);
@@ -172,7 +172,7 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
      *   asking the caller for blinding values directly and expecting them to retry on failure.
      */
     memcpy(keydata, nonce32, 32);
-    if (seed32) {
+    if (seed32 != NULL) {
         memcpy(keydata + 32, seed32, 32);
     }
     secp256k1_rfc6979_hmac_sha256_initialize(&rng, keydata, seed32 ? 64 : 32);