libvirt-sandbox-config: Don't deref NULL
libvirt-sandbox-config: Don't deref NULL

The problem is in gvir_sandbox_config_add_mount_opts. When
parsing disk string, "format=" may be within it. This is
supposed to change disk format from raw to the desired one.
However, due to bug in our implementation, we may end up
dereferencing a NULL pointer.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian