e1_input: Guard against crash by users calling reentrant e1inp_sign_link_destroy()Related: osmo-bsc.git Change-Id Ice71b3143f167482e4a23759012b58e9ee13bfc0Related: osmo-bts.git Change-Id Ibc06cdc2d2cd2028b7676fa0c3211ae251cca587Change-Id: Ib761cece68f1d809e27194bc5cc2eb15ad2ae4eb