xAODMissingET: Protect against out-of-bounds shift.
In MissingETAssociationHelper::objSelected, check that objIdx is not outside of the possible bounds for a bit shift. This can happen in the objSelected overload taking an IParticle* is called, and the particle is not found. In that case, objIdx will be -1.
Cleans up an error seen from the undefined behavior sanitizer.