examples/mdev.conf.change_blockdev.sh: update

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
mdev: call index_in_strings on $ACTION only after we checked it for NULL

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox