`mrb_dup()` should fail if `fd` is a negative integer; ref #4005

mrbgems/mruby-io/src/io.c

@@ -542,12 +542,12 @@ mrb_dup(mrb_state *mrb, int fd, mrb_bool *failed)
 {
   int new_fd;
 
-  *failed = FALSE;
+  *failed = TRUE;
   if (fd < 0)
     return fd;
 
   new_fd = dup(fd);
-  if (new_fd == -1) *failed = TRUE;
+  if (new_fd > 0) *failed = FALSE;
   return new_fd;
 }