st/mesa: an incomplete texture may have a zero-size first image
Fixes a regression introduced by commit 42624ea8 which triggered an assertion in dEQP-GLES2.functional.texture.completeness.cube.not_positive_level_0 While stImage must have a non-zero size as verified by the caller, we also look at the size of the base image in an attempt to make a better guess at the level0 size (this is important when the base image size is odd). However, the base image may have a zero size even when it exists. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96629 Cc: 12.0 <mesa-stable@lists.freedesktop.org> Reviewed-by:Ilia Mirkin <imirkin@alum.mit.edu> (cherry picked from commit 0ba053b3)