mesa: recover target_check before get_current_tex_objects

At compressed_tex_sub_image we only can obtain the tex_object after
compressed_subtexture_target_check is validated for TEX_MODE_CURRENT.
So if the target is wrong the error is raised to the user.

This completes the fix for the regression introduced on "mesa: refactor
compressed_tex_sub_image function" of the pending failing tests:

dEQP-GLES3.functional.negative_api.texture.compressedtexsubimage3d
dEQP-GLES31.functional.debug.negative_coverage.get_error.texture.compressedtexsubimage3d

v2: Fix warning that texObj might be used uninitialized (Gert Wollny)

Fixes: 7df233d6 ("mesa: refactor compressed_tex_sub_image function")
Reviewed-By: Gert Wollny <gert.wollny@collabora.com>
(cherry picked from commit 74a7e3ed)