Replacement ifquery doesn't fail for unconfigured bridge

[sshambar]

When mstp_config_bridge calls ifquery, it checks the return code to see if a bridge has been configured. However, the replacement ifquery program always succeeds (but returns an empty string).

A fix could either be to either:

• make ifquery fail for unconfigured bridges
• update mstp_config_bridge to check the $? as well that a value was returned.
• both :)

I'm happy to supply a patch for any of the above if you think it's something useful.