wlan: Validate pkt_stats_buffers before dereferencing

For wlan logging, number of pkt_stats_buffers are stored
in pkt_stat_num_buf. If skb allocation failed pkt_stats_buffers
is freed but pkt_stat_num_buf is not set zero. This causes
NULL pointer dereference while deactivating wlan logging.

To avoid this set pkt_stat_num_buf to zero if skb allocation fails
and validate pkt_stats_buffers before de-referencing.

Change-Id: I54f8167e21fb71662c94988f9cd8033363611e16
CRs-Fixed: 2708247