net: context: fix net context / net conn leak

If a net context is not connected or listening, we can go ahead and
call net_context_unref() to free it up instead of waiting for
FIN_ACK which will never happen.

Change-Id: Ice06f572df64f2edb5918c10c92087ce0b7b254a
Signed-off-by: Michael Scott <michael.scott@linaro.org>