pack.c: fix underflow

* pack.c (pack_unpack_internal): get rid of underflow.
  https://hackerone.com/reports/298246

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62975 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 1 insertions, 1 deletions

diff --git a/pack.c b/pack.c
index 45b816a590..12f30cb6b0 100644
--- a/pack.c
+++ b/pack.c
@@ -1127,7 +1127,7 @@ pack_unpack_internal(VALUE str, VALUE fmt, int mode)
 	else if (ISDIGIT(*p)) {
 	    errno = 0;
 	    len = STRTOUL(p, (char**)&p, 10);
-	    if (errno) {
+	    if (len < 0 || errno) {
 		rb_raise(rb_eRangeError, "pack length too big");
 	    }
 	}