mwifiex: restore handling of NULL parameters

Prior to a5ffddb70c5cab "mwifiex: remove casts of void pointers" the
code assumed that the data_buf parameter could be a NULL pointer.
The patch preserved some NULL checks but not consistently, so there
was a potential for NULL dereferences and it changed the behavior.
This patch restores the original behavior.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>