msm: vidc: Fix scratch buffer allocation.

In some cases, firmware is reqesting driver
to allocate one scratch with size zero,
eg: if height and width is set to zero on
firmware. This change handles this scenario
appropriately to avoid kernel panic.

Change-Id: If670f91ef600678af8e246934f685defcb7fdfeb
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>