addressbook: Fix cancellable handling if not in a transaction
addressbook: Fix cancellable handling if not in a transaction

If not in a transaction, ebsql_exec() will set a temporary GCancellable
for the operation (transactions set their own), but would then
erroneously clear *all* cancellables after the query finished; even
those set by transactions.

Coverity issue: #1214491

https://bugzilla.gnome.org/show_bug.cgi?id=730374

matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server