qcacmn: Add null pointer check in qdf_dp_trace

In hdd_tx_timeout(), null nbuf is passed to qdf_dp_trace(). This
results a null pointer dereference when getting data buffer.
Since data buffer is not mandory for qdf_dp_add_record(), add null
nbuf check in qdf_dp_trace().

Change-Id: If5fb4753d382d8f29bdf4d934a7910a28a7c76aa
CRs-Fixed: 2244543
1 file changed