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: Ibc06cdc2d2cd2028b7676fa0c3211ae251cca587