Fix sign mismatch and cast to bool.

Author: evoskuil

src/secp256k1.c

@@ -270,7 +270,7 @@ int secp256k1_ec_pubkey_serialize(const secp256k1_context* ctx, unsigned char *o
 
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(outputlen != NULL);
-    ARG_CHECK(*outputlen >= ((flags & SECP256K1_FLAGS_BIT_COMPRESSION) ? 33 : 65));
+    ARG_CHECK(*outputlen >= ((flags & SECP256K1_FLAGS_BIT_COMPRESSION) != 0 ? 33u : 65u));
     len = *outputlen;
     *outputlen = 0;
     ARG_CHECK(output != NULL);