glsl: Fix hypothetical NULL dereference in ast_process_structure_or_interface_block

glsl: Fix hypothetical NULL dereference in ast_process_structure_or_interface_block

Fixes issue identified by Klocwork analysis:

    Pointer 'field_type' returned from call to function 'glsl_type' at
    line 4126 may be NULL and may be dereferenced at line 4139.  Also
    there are 2 similar errors on line(s) 4165, 4174.

In practice, it should be impossible to actually get NULL in here
because a syntax error would have already caused compilation to halt.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>