libxt_TOS: make sure --set-tos value/mask is recognized

Only when a 'stop' pointer is passed, the string may consist of more
than just a number.

Reported-by: Anonymous
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/extensions/tos_values.c b/extensions/tos_values.c
index 1630834..4c0b0bc 100644
--- a/extensions/tos_values.c
+++ b/extensions/tos_values.c
@@ -56,8 +56,9 @@
 {
 	const unsigned int max = 255;
 	const struct tos_symbol_info *symbol;
+	char *tmp;
 
-	if (strtonum(str, NULL, NULL, 0, max))
+	if (strtonum(str, &tmp, NULL, 0, max))
 		return tos_parse_numeric(str, tvm, max);
 
 	/* Do not consider ECN bits */