gallium/osmesa: Fix the inability to set no context as current.

Currently there is no way to make no context current w/gallium + osmesa.
The non-gallium version of osmesa does this if the context and buffer
passed to `OSMesaMakeCurrent` are both null. This small change makes it
so that this is also the case with the gallium version.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Hal Gentz <zegentzy@protonmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 57c89433)