mesa: Fix FB blitting in case of zero size src or dst rect

mesa: Fix FB blitting in case of zero size src or dst rect

Framebuffer blitting operation should be skipped if any of the
dimensions (width/height) of src/dst rect is zero.

V2: Move the dimension check after error checking in _mesa_BlitFramebuffer.

Fixes: fbblit(negative.nullblit.zeroSize) in Intel oglconform
https://bugs.freedesktop.org/show_bug.cgi?id=59495

Note: Candidate for all the stable branches.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>