glsl: Fix hypothetical NULL dereference related to process_array_type

glsl: Fix hypothetical NULL dereference related to process_array_type

Ensure that process_array_type never returns NULL, and let
process_array_type handle the case where the supplied base type is NULL.

Fixes issues identified by Klocwork analysis:

    Pointer 'type' returned from call to function 'get_type' at line
    1907 may be NULL and may be dereferenced at line 1912.

and

    Pointer 'field_type' checked for NULL at line 4160 will be
    dereferenced at line 4165. Also there is one similar error on line
    4174.

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