fix underflow

Merged: https://github.com/ruby/ruby/pull/15763

pack.c

@@ -302,7 +302,7 @@ pack_pack(rb_execution_context_t *ec, VALUE ary, VALUE fmt, VALUE buffer)
         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");
             }
         }