mesa/image: assert on bad format

mesa/image: assert on bad format

NULL as an error indicator is meaningless, since it will return NULL
on success anyway if the caller passes in zero as the image's address
and asks to calculate the offset of the first pixel. For example,
_mesa_validate_pbo_access() does this.

This also matches the code in the non-GL_BITMAP codepath, which
already has an assert like this.

v2: Per Brian Paul's review, remove the function call entirely
and tighten the assert to only accept the two formats compatible with
GL_BITMAP. They always have one component per pixel.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>