cosmetic: log error when using a conn that's in release

If a conn is attempted to be used when in release, log an error, but don't skip
tracking.

No current code path apparently hits this, according to msc_vlr_tests. Just
making sure that we will prominently see such errors when we introduce any.

Change-Id: I8dd20ee56ce5ad7a90fcd03a06604c383e5eed54
diff --git a/src/libmsc/osmo_msc.c b/src/libmsc/osmo_msc.c
index d55940d..2a868a8 100644
--- a/src/libmsc/osmo_msc.c
+++ b/src/libmsc/osmo_msc.c
@@ -349,7 +349,10 @@
 	OSMO_ASSERT(conn);
 
 	if (conn->in_release)
-		return NULL;
+		LOGPSRC(DREF, LOGL_ERROR, file, line,
+			"%s: MSC conn use error: using conn that is already in release (%s)\n",
+			vlr_subscr_name(conn->vsub),
+			msc_subscr_conn_use_name(balance_token));
 
 	if (balance_token != MSC_CONN_USE_UNTRACKED) {
 		uint32_t flag = 1 << balance_token;