qcacmn: Fix null pointer dereference in extract_sar_limit_event_tlv

When WMI_SAR_GET_LIMITS_EVENTID is received from firmware, the
function extract_sar_limit_event_tlv is called to update the SAR
limits for all the chains of each band. There is a for loop
defined to loop over each item in param_buf->sar_get_limits.
Since the param_buf->sar_get_limits could be either optionally
defined or not a part of the message at all there is a potential
NULL pointer dereference if sar_get_limits is not sent as part
of the WMI_SAR_GET_LIMITS_EVENTID event.

param_buf->sar_get_limits needs to be checked for NULL prior to
derefencing it.

Change-Id: I93c07fa8048df97c6f6960b0db6df3bbc30e23b4
CRs-Fixed: 2336928