ar5523: Don't dereference sta if NULL
ar5523: Don't dereference sta if NULL

A missing else caused a potential NULL dereference.

Reported-by: Yuanhan Liu <5dcef21195e64c9c08aa779bb62b25e9133eaa33@linux.intel.com>
Signed-off-by: Pontus Fuchs <a5605b7eadbbe8624843094042f247faf6b65fe5@gmail.com>
Signed-off-by: John W. Linville <2a53bac7a5d324865ef46ec4c38b2c0fba1456b4@tuxdriver.com>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas