commit: Add NULL check for overlay_bs

commit: Add NULL check for overlay_bs

I can't see how overlay_bs could become NULL with the current code, but
other code in this function already checks it and we can make Coverity
happy with this check, so let's add it.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit b1e1fa0c3afc7f671fbc24645bdf67949a5657e5)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>