glsl: Fix null return coverity warning

glsl: Fix null return coverity warning

Add an assert on the result of as_dereference() not being NULL:

>>>     CID 1324978:  Null pointer dereferences  (NULL_RETURNS)
>>>     Dereferencing a null pointer "deref_record->record->as_dereference()".

Since we are introducing a new variable to hold the result of
as_dereference(), take the opportunity to rename deref_record_type to
interface_type and just name the new variable interface_deref, which is
less confusing.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>