Make this no-op variant of fmtcheck() compatible with the libc version

when called with NULL as first argument.

distrib/utils/libhack/fmtcheck.c

@@ -1,7 +1,7 @@
-/*	$NetBSD: fmtcheck.c,v 1.1 2019/06/22 22:50:39 christos Exp $	*/
+/*	$NetBSD: fmtcheck.c,v 1.2 2022/04/14 15:51:29 martin Exp $	*/
 #include <stdio.h>
 const char *
 fmtcheck(const char *a, const char *b)
 {
-	return a;
+	return a ? a : b;
 }