fix Classmark Update without VLR subscriber

This recent patch moves Classmark storage to the VLR subscriber, and introduced
a segfault when a Classmark Update is received during IMSI detach:

commit 986fe7ed18580775bed91399a1f02eae60bda251
change-id I27081bf6e9e017923b2d02607f7ea06beddad82a
Mon Sep 17 01:12:13 2018 +0200
"store classmark in vlr_subscr, not conn"

It assumed that we would never accept any Classmark Update messages unless we
also have a valid subscriber for it. Well, that is proven wrong by the
ttcn3-msc-test TC_imsi_detach_by_imsi(), which brings osmo-msc to its knees.

Fix: in case of no valid vlr_subscr being present, store Classmark in the conn
temporarily, and copy any received Classmark to VLR subscriber as soon as it
gets associated with the conn (if at all).

Change-Id: Ib2a2ae6bf86e8f29fc6751a8b5cdb7187cd70290
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 70ac934..54026f6 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -181,6 +181,10 @@
 
 		bool waiting_for_clear_complete;
 	} a;
+
+	/* Temporary storage for Classmark Information for times when a connection has no VLR subscriber
+	 * associated yet. It will get copied to the VLR subscriber upon msc_vlr_subscr_assoc(). */
+	struct gsm_classmark temporary_classmark;
 };