dh-gex: Use SSH_STRING_FREE()

Fixes T183

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

src/dh-gex.c

@@ -265,7 +265,7 @@ static SSH_PACKET_CALLBACK(ssh_packet_client_dhgex_reply)
     }
 
     rc = ssh_dh_import_next_pubkey_blob(session, pubkey_blob);
-    ssh_string_free(pubkey_blob);
+    SSH_STRING_FREE(pubkey_blob);
     if (rc != 0) {
         goto error;
     }