Show NULL when NULL char

This might otherwise confuse with regular '0'.

print_helper.c

@@ -29,7 +29,7 @@ void print_char(unsigned char c)
 {
 	switch (c) {
 	case 0:
-		printf("0\n");
+		printf("NULL\n");
 		break;
 	case '\n':
 		printf("\\n\n");