fix failure of mbsinit(0) (not UB; required to return nonzero)

issue reported by Richard Pennington; slightly simpler fix applied

src/multibyte/mbsinit.c

@@ -13,5 +13,5 @@
 
 int mbsinit(const mbstate_t *st)
 {
-	return !*(unsigned *)st;
+	return !st || !*(unsigned *)st;
 }