fix vlr ops.subscr_assoc re-association

In rare cases, a conn is already associated with a subscriber. So far, we
abort()ed on that, bringing the entire osmo-msc down. Rather log an error and
keep the service running.

In vlr.ops.subscr_assoc, add success/failure return value, and abort the
LU/PARQ on error.

I haven't figured out in detail yet why/how a subscriber would re-launch a
LU/PARQ on a conn that is already associated, so far it is merely clear that we
do not want to crash the MSC if that happens. A log is in OS#3742.

Related: OS#3742, OS#3743
Change-Id: Ic0d54644bc735700220b1ef3a4384c217d57d20f
4 files changed