mesa: Fix error in target validation of glCompressedTex(ture)SubImage3D() calls

mesa: Fix error in target validation of glCompressedTex(ture)SubImage3D() calls

Basically, two different target error checks are chained consecutively, and the
second one is executed regardless the result of the first one. This produces an
incorrect error if the first check fails but is overrided by the second.

This patch conditions the execution of the second check to a successful pass of
the first one.

Fixes 1 dEQP test:
* dEQP-GLES3.functional.negative_api.texture.compressedtexsubimage3d

Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>