selftests/nolibc: avoid buffer underrun in space printing

If the test description is longer than the status alignment the
parameter 'n' to putcharn() would lead to a signed underflow that then
gets converted to a very large unsigned value.
This in turn leads out-of-bound writes in memset() crashing the
application.

The failure case of EXPECT_PTRER() used in "mmap_bad" exhibits this
exact behavior.

Fixes: 29f5540b ("selftests/nolibc: add EXPECT_PTREQ, EXPECT_PTRNE and EXPECT_PTRER")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>

tools/testing/selftests/nolibc/nolibc-test.c

@@ -151,7 +151,8 @@ static void result(int llen, enum RESULT r)
 	else
 		msg = "[FAIL]";
 
-	putcharn(' ', 64 - llen);
+	if (llen < 64)
+		putcharn(' ', 64 - llen);
 	puts(msg);
 }