r600g/sb: Handle undef in read port tracker
e8e443 missed adding check for undef values also in unreserve function, leading to an assert triggering. Signed-off-by:Glenn Kennard <glenn.kennard@gmail.com> Cc: <mesa-stable@lists.freedesktop.org> Signed-off-by:
Dave Airlie <airlied@redhat.com> (cherry picked from commit 36f1999a)