net: app: Check if underlying net_context is freed

It might happen in TCP client, that the TCP connection is terminated
in which case net_context is freed. Check this and mark corresponding
net_context inside net_app to NULL. This way there will be no issue
to access already freed net_context.

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