Changes

abis: Fix reusing link->conn while it is being destroyed

Call to osmo_stream_cli_destroy() may end up triggering disconnect_cb(),
and we don't want to attempt doing more stuff with that conn when that
happens (we were even trying to reconnect() it...).
Avoid it by marking the pointer as NULL immediatelly before calling
e1inp_sign_link_destroy().

Change-Id: I56605766af974d86610d0958f4b82e6e4ff94502