slim_ngd: Add NULL pointer check apart from error check

Check against NULL pointer after allocating
the memory for msm_slim_ctrl structure pointer
dynamically to avoid unexpected crashes.

Earlier this pointer is checked only for error
codes. In this patch NULL pointer check is also
added.

CRs-Fixed: 562417
Change-Id: I5cd731144f88b89ee412d19f110b3df6426186f7
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>