Use the correct MTU for prev_path

Co-Authored-By: Benjamin Saunders <ben.e.saunders@gmail.com>

quinn-proto/src/connection/mod.rs

@@ -480,7 +480,7 @@ impl Connection {
                     SpaceId::Data,
                     "PATH_CHALLENGE queued without 1-RTT keys"
                 );
-                buf.reserve(self.path.current_mtu() as usize);
+                buf.reserve(MIN_INITIAL_SIZE as usize);
 
                 let buf_capacity = buf.capacity();