mesa: Skip texstore for 0-sized texture data.

mesa: Skip texstore for 0-sized texture data.

The intel driver (and gallium, it looks like, though it doesn't use
these texstore functions at this point) doesn't bother making storage
for textures with 0 width, height, or depth.  This avoids them having
to deal with returning a mapping for that nonexistent data.

Fixes assertion failures with an upcoming intel driver change.

Reviewed-by: Brian Paul <brianp@vmware.com>