net: Print error if buf or interface are NULL when receiving pkt

Although very unlikely, I saw crashes at startup that could
be because of the buf or iface were null.

Change-Id: I8649eeb4f24fcd7d9f2acaeefaba4e6593388e91
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>