media: staging: atomisp: sh_css_calloc shall return a pointer to the allocated space

The calloc function returns either a null pointer or a pointer to the
allocated space. Add the second case that is missed.

Fixes: da22013f

 ("atomisp: remove indirection from sh_css_malloc")
Signed-off-by: Sergei A. Trusov <sergei.a.trusov@ya.ru>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>