net: 6lo: ieee802154: Do no try to unref NULL net_buf

It is possible that the cached buf is NULL so avoid nasty
error print by not trying to free NULL pointer.

Change-Id: Iec53164f6ffab4b830aba63a1f351ca2349a43ff
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>