net: tcp: Reset context->tcp to NULL after net_tcp_release.

net_context_unref() is protocol agnostic, after being un-referenced,
the same context might end up being used then for UDP,
if context->tcp is not reset to NULL, calling net_context_unref()
after this UDP usage will again try to release this TCP pointer
which might lead to random error.

Signed-off-by: june li <junelizh@foxmail.com>