glsl: Fix forward NULL dereference coverity warning

glsl: Fix forward NULL dereference coverity warning

The comment says that it should be impossible for decl_type to be NULL
here, so don't try to handle the case where it is, simply add an assert.

>>>     CID 1324977:  Null pointer dereferences  (FORWARD_NULL)
>>>     Comparing "decl_type" to null implies that "decl_type" might be null.

No piglit regressions observed.

Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>