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

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>
(cherry picked from commit 6702c1581092bce230742d02ebf9325f68bd247a)