libcrypto: fix resource leak in hmac_final

Fix a resource leak in `hmac_final`: say `HMAC_CTX_free` instead
of `HMAC_CTX_reset`.  This matches the error handling as done in
`hmac_init`.  Introduced with cf1e808e2f.

The problem is reproducible running the `pkd_hello` test with:

    valgrind --leak-check=full ./pkd_hello -i1 -t torture_pkd_openssh_dsa_rsa_default

Resolves https://red.libssh.org/issues/252.

Cherry-picked from a64ddff3fe

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/libcrypto.c

@@ -366,7 +366,8 @@ void hmac_final(HMACCTX ctx, unsigned char *hashmacbuf, unsigned int *len) {
   HMAC_Final(ctx,hashmacbuf,len);
 
 #ifndef OLD_CRYPTO
-  HMAC_CTX_reset(ctx);
+  HMAC_CTX_free(ctx);
+  ctx = NULL;
 #else
   HMAC_cleanup(ctx);
 #endif