Not all frames are read if more than one is received via tcp

During a tunneling connection (via TCP) to a KNX IP interface (MDT SCN-IP000.03), it sometimes happens that I receive more than one frame at once from the TCP socket. In this case, only the first frame is processed, and the second frame remains in the rxbuffer. It is only being processed once the next tcp read occurs, which in turn leaves the most recent frame in the buffer.

The responsible code is in qknxnetipendpointconnection.cpp, lines 575ff. I attached a patch for qt 5.15.2 with the fix that I did to solve this issue for now.