sched: AST_SCHED_REPLACE_UNREF can lead to use after free of data

The data can be freed if the old object '_data' is the same object as
new 'data'. Because at first the object is unreferenced which can lead
to destroying it.

This could happened in res_pjsip_pubsub when the publication is updated
which could lead to segfault in function publish_expire.

Change-Id: I0164f57c387243510bdbd2f8dcf33377b6c202da