wifi: ath12k: Add NULL check to validate tpc_stats

wifi: ath12k: Add NULL check to validate tpc_stats

While processing TPC stats received from firmware, there are chances that
the tpc_stats might not be filled and the data is not available. This can
happen under two scenarios. First, when firmware sends a non-zero event
count before event count 0. When this happens, tpc_stats will be checked
for data before memory allocation and the tpc_stats will be unavailable.
Second, when memory allocation failed when event count received is 0 and
the firmware still sends a non-zero event. When this happens, memory will
not be allocated for tpc_stats though event count is 0, so when non-zero
event count is received, tpc_stats will be empty. There are checks to
validate if tpc_stats variable is filled that are used in two subsequent
places, but these are placed after tpc_stats is dereference without
checking if it is NULL or has valid data.

Fix this by removing the mentioned checks and adding a NULL check after
assigning tpc_stats to check if it is valid.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-QCAHKSWPL_SILICONZ-1

Closes: https://scan7.scan.coverity.com/#/project-view/52668/11354?selectedIssue=1637145
Fixes: f0c3bb78e42f ("wifi: ath12k: Add Support to Parse TPC Event from Firmware")
Signed-off-by: Roopni Devanathan <quic_rdevanat@quicinc.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250221041250.769491-1-quic_rdevanat@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>