[HL2MP] Check for physics validation when using the physcannon

[speedvoltage]

Issue:
When you use the physcannon and the physics object you're holding gets motion-disabled, it might not let go of the object. Plus, there's no proper verification to see if a physics object is present before trying to perform actions like throwing it with the primary attack.

Fix:

• Added a check to make sure that if a physics object becomes motion-disabled, the physcannon will release it right away.
• Added a validation for pPhys. Once that's confirmed, we can proceed with the usual logic for the primary attack check.