Fix a couple of warnings of errors in MSVC builds

Two warnings (in tests & examples) in particular would cause problems:
bad format causing invalid data output or a bad chdir due to out of
scope buffer use.

example/ssh2_agent_forwarding.c

@@ -225,7 +225,6 @@ int main(int argc, char *argv[])
     }
     for(;;) {
         /* loop until we block */
-        int rc;
         do {
             char buffer[0x4000];
             rc = libssh2_channel_read(channel, buffer, sizeof(buffer) );