glsl: Check that layout is non-null before dereferencing.

layout should only be null for structs, but it's checked everywhere else
and confuses Coverity (CID 1358495).

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/compiler/glsl/ast_to_hir.cpp

@@ -6818,7 +6818,7 @@ ast_process_struct_or_iface_block_members(exec_list *instructions,
           * the structure may contain a structure that contains ... a matrix
           * that need the proper layout.
           */
-         if (is_interface &&
+         if (is_interface && layout &&
              (layout->flags.q.uniform || layout->flags.q.buffer) &&
              (field_type->without_array()->is_matrix()
               || field_type->without_array()->is_record())) {