mwifiex: corner case NULL pointer dereference fix

When next scan command is delayed due to Tx traffic and
meanwhile synchronous command is received followed by a signal,
we cance all pending commands. NULL pointer dereference is seen
in this case while queueing next command in scan delay timer.
This patch adds a check to fix this issue.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>