vl: Silence a warning at -Og

vl: Silence a warning at -Og

   ../src/gallium/auxiliary/vl/vl_winsys_dri.c: In function ‘vl_dri2_screen_texture_from_drawable’:
   ../src/gallium/auxiliary/vl/vl_winsys_dri.c:239:34: warning: ‘back_left’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     239 |    dri2_handle.stride = back_left->pitch;

The server would be broken if it failed to inform us of the back left
buffer, but let's leave nothing to chance.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>