sde: rotator: Add null pointer checks

Pointers in some cases are dereferenced even after
freeing memory region pointed by them. Assign NULL to
free pointers and perform proper NULL pointer checks.

Change-Id: Id9caef8af0c9d06e40e99ed71db68197fc6cf4f4
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>