Ignore NULL pointers in vmod.to{upper|lower}()

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5410 d4fa192b-c00b-0410-8231-f00ffab90ce4

lib/libvmod_std/vmod_std.c

@@ -55,11 +55,13 @@ vmod_updown(struct sess *sp, int up, const char *s, va_list ap)
 	e += u;
 	p = s;
 	while (p != vrt_magic_string_end && b < e) {
-		for (; b < e && *p != '\0'; p++)
-			if (up)
-				*b++ = toupper(*p);
-			else
-				*b++ = tolower(*p);
+		if (p != NULL) {
+			for (; b < e && *p != '\0'; p++)
+				if (up)
+					*b++ = toupper(*p);
+				else
+					*b++ = tolower(*p);
+		}
 		p = va_arg(ap, const char *);
 	}
 	if (b < e)