qcacld-3.0: fix code defects for RRM frame processing

Directly dereferencing pointer beacon_xmit_ind before
null-checking, which may result in null pointer issue.
Meanwhile, measurement_idx should be checked to avoid
out-of-bounds read.
Fix is to check before dereferencing it.

CRs-Fixed: 2751550
Change-Id: I5241b3b16fcd9a04da121fd938b27d4070ea4e06