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>