io.c (rb_update_max_fd): fail with a negative file descripter

Coverity Scan points out that ext/socket/unixsocket.c may pass -1 to
rb_update_max_fd.  I'm unsure whether it can happen actually or not, but
it would be good for the function to reject a negative value.

io.c

@@ -201,7 +201,7 @@ rb_update_max_fd(int fd)
     rb_atomic_t max_fd = max_file_descriptor;
     int err;
 
-    if (afd <= max_fd)
+    if (fd < 0 || afd <= max_fd)
         return;
 
 #if defined(HAVE_FCNTL) && defined(F_GETFL)