qcacmn: Check peer before accessing vdev in tx_comp_handler

Race Condition is observed as tx_comp_handler is
accessing vdev pointer while its getting freed up
parallelly.
To fix this, peer value is checked before accessing vdev.
If peer	is null, vdev is not accessed and the function is
returned.

Change-Id: I05c93c54e2dbb3f40c5c053591d2d66c6143f656
CRs-Fixed: 2708390