Use a local variable and check the record payload length validity before
writing it into record->payload_length in hopes of getting rid of a
bogus static analyzer warning. The negative return value was sufficient
to avoid record->payload_length being used, but that seems to be too
complex for some analyzers. (CID 122668)
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>