make msc_a_vsub() and others NULL-safe

Fixes: coverity CID 198451
Change-Id: Icd146ae512236a09cad080ed3eb85944e8f5cee4
diff --git a/src/libmsc/msc_t.c b/src/libmsc/msc_t.c
index ec5531f..c6bba47 100644
--- a/src/libmsc/msc_t.c
+++ b/src/libmsc/msc_t.c
@@ -902,6 +902,8 @@
 
 struct vlr_subscr *msc_t_vsub(const struct msc_t *msc_t)
 {
+	if (!msc_t)
+		return NULL;
 	return msub_vsub(msc_t->c.msub);
 }