mesa: fix invalid target error handling for teximage

This commit moves the target check before using _mesa_get_current_tex_object
to fix a "Mesa implementation error: bad target in _mesa_get_current_tex_object()"
error.

Fixes: 9dd1f7ce ("mesa: pass gl_texture_object as arg to not depend on state")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 16233797)