Changes

abis: Fix reusing bts->*_link while it is being destroyed

Call to e1inp_sign_link_destroy() may trigger a sign_down() callback,
which if happening synchronously, could end up reentring the same code
path we are in before bts->*_link was set to NULL.
Avoid it by marking the pointer as NULL immediatelly before calling
e1inp_sign_link_destroy().

Change-Id: Ice71b3143f167482e4a23759012b58e9ee13bfc0