vbox: Address false positive for NULL dereference

Resolve a false positive from 'vboxIIDFromUUID_v2_x()'. The code sets
'iid->value = &iid->backing' unconditionally prior to calling 'nsIDFromChar()'.
The 'vboxIIDUnalloc_v2_x()' checks iid->value to not be &iid->backing. The
iid->backing is a static buffer within the initialized structure.

src/vbox/vbox_tmpl.c

@@ -444,6 +444,7 @@ vboxIIDFromUUID_v2_x(vboxGlobalData *data, vboxIID_v2_x *iid,
 
     iid->value = &iid->backing;
 
+    sa_assert(iid->value);
     nsIDFromChar(iid->value, uuid);
 }