st/va: handle default post process regions
st/va: handle default post process regions

Avoid referencing NULL pointers.

Signed-off-by: Christian König <c7ea837d7a46effe4232b086213468b8b31643bf@amd.com>
Reviewed-by: Julien Isorce <2904e845d63d102574cbb95a4bcdefac0a5d7af0@samsung.com>
Tested-by: Julien Isorce <2904e845d63d102574cbb95a4bcdefac0a5d7af0@samsung.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler