make msc_a_vsub() and others NULL-safe

Fixes: coverity CID 198451
Change-Id: Icd146ae512236a09cad080ed3eb85944e8f5cee4
diff --git a/src/libmsc/msc_i.c b/src/libmsc/msc_i.c
index 6badba6..05df9cb 100644
--- a/src/libmsc/msc_i.c
+++ b/src/libmsc/msc_i.c
@@ -332,6 +332,8 @@
 
 struct vlr_subscr *msc_i_vsub(const struct msc_i *msc_i)
 {
+	if (!msc_i)
+		return NULL;
 	return msub_vsub(msc_i->c.msub);
 }