Officer investigating report of man with a gun entitled to qualified immunity for shooting and killing man who failed to stop in response to commands, verbally challenged officers, concealed hands in front of waistband and then lifted shirt with left hand and quickly withdrew right hand from waist band; officer could reasonably believe he was reaching for a gun although as it turned out, man was unarmed; court states, “regarding such circumstances, the Fourth Amendment is clear: officers need not wait until they see the gun’s barrel or the knife’s blade before using deadly force to protect