abis.c: Fix segfault on OML link loss
abis.c: Fix segfault on OML link loss

When the OML signalling link is lost, first set bts->oml_link = NULL,
then iterate over the RSL links and close them.  Closing the RSL link
will cause a OML state change message to be sent, which in turn tries
to use the no-longer-existing OML link.

The code should be cleaned up further to distinguish which signalling
link was lost, and actually communicate a RSL(only) loss to OML.

But for now, it's best to simply close down all links and terminate
osmo-bts to ensure all state is properly reset and recovered.

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts