Add some small fixes to the MQAC local exploit

This PR piggybacks on to @zeroSteiner's great work on the MQAC local exploit.

* Add a check for `INVALID_HANDLE_VALUE` when attempting to open the device, as this is what is returned when the device doesn't exist (ie. when MSMQ is not installed).
* Make sure that we only run the exploit against targets that we support directly to make sure we don't BSOD machines (such as what happens with SP1/SP2).
* Add a call to `check` in the exploit code to cover all bases.

Minor fixes really, just to make sure that we don't lose boxes when running the sploit. Cheers!
If the target system is non-Windows XP and the MQAC service is installed, ```CheckCode::Safe``` is returned and the exploit finishes without printing anything useful to the user.