Use size_t instead of int for RFC6979 outlen copy

If outlen is > INT_MAX, could trigger segfault or hang after copy
int now = outlen.

src/hash_impl.h

@@ -265,7 +265,7 @@ static void secp256k1_rfc6979_hmac_sha256_generate(secp256k1_rfc6979_hmac_sha256
 
     while (outlen > 0) {
         secp256k1_hmac_sha256 hmac;
-        int now = outlen;
+        size_t now = outlen;
         secp256k1_hmac_sha256_initialize(&hmac, rng->k, 32);
         secp256k1_hmac_sha256_write(&hmac, rng->v, 32);
         secp256k1_hmac_sha256_finalize(&hmac, rng->v);