megaraid: Fix possible NULL pointer deference in mraid_mm_ioctl

This adds the needed check after the call to the function
mraid_mm_alloc_kioc in order to make sure that this function has not
returned NULL and therefore makes sure we do not deference a NULL
pointer if one is returned by mraid_mm_alloc_kioc.  Further more add
needed comments explaining that this function call can return NULL if
the list head is empty for the pointer passed in order to allow furture
users to understand this required pointer check.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Acked-by: Sumit Saxena <sumit.saxena@avagotech.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>