backend: Check number of fds before parsing message

This prevents an fd from being consumed if the message will later fail
to parse because another fd has been read. Which is theoretically
possible.

wayland-backend/src/rs/wire.rs

@@ -192,6 +192,11 @@ pub fn parse_message<'a>(
         return Err(MessageParseError::MissingData);
     }
 
+    let fd_len = signature.iter().filter(|x| matches!(x, ArgumentType::Fd)).count();
+    if fd_len > fds.len() {
+        return Err(MessageParseError::MissingFD);
+    }
+
     let (mut payload, rest) = raw.split_at(len);
     payload = &payload[2..];