glsl: Check that layout is non-null before dereferencing.
glsl: Check that layout is non-null before dereferencing.

layout should only be null for structs, but it's checked everywhere else
and confuses Coverity (CID 1358495).

Reviewed-by: Timothy Arceri <5dd83789ef3714029244367ba1ca09f1d7627408@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler