st/mesa: an incomplete texture may have a zero-size first image

st/mesa: an incomplete texture may have a zero-size first image

Fixes a regression introduced by commit 42624ea83 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>