drivers: slip: Fix memory corruption when receiving packet

It is possible that the slip->last pointer is not set during
the startup of the slip driver if we receive some garbage bytes.
This in turn caused memory to be overwritten in random places.

Jira: ZEP-1737

Change-Id: Ia2ce193b81a8f707b7ca63497d497ea70699bb32
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>