glsl/ast: don't crash when func_name is NULL

This fixes a crash in
GL43-CTS.shader_subroutine.subroutines_not_allowed_as_variables_constructors_and_argument_or_return_types

If we can't find the func_name in one of these paths,
we have emitted an earlier error so just return here.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/compiler/glsl/ast_function.cpp

@@ -2078,6 +2078,10 @@ ast_function_expression::hir(exec_list *instructions,
          func_name = id->primary_expression.identifier;
       }
 
+      /* an error was emitted earlier */
+      if (!func_name)
+         return ir_rvalue::error_value(ctx);
+
       ir_function_signature *sig =
 	 match_function_by_name(func_name, &actual_parameters, state);