net: context: Fixing bug in net context put flow

Following flow does not work:
net_context_get
net_context_bind
..
..
net_context_put
net_context_get
net_context_bind

At instance of call to net_context_bind, conn_handler
is not NULL and returns with EISCONN.

This patch sets conn_handler to NULL in net_context_unref

Change-Id: I56a50839101b22161644b3cd7c5f510fa1abae3e
Signed-off-by: Mitul Shah <mitul.a.shah@intel.com>