Occasional crash when executing M552 S0

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If M552 S0 is commanded while the network is active, very occasionally the firmware crashes in function NetworkResponder::SendData at line outBuf = OutputBuffer::Release(outBuf) because outbuf has become null even though it was checked to be non-null earlier in the function. This can be reproduced by executing M552 S1 and M552 S0 in a loop, with appropriate G4 delays between them (e.g. 7 seconds after M442 S1 and 2 seconds after M552 S0).

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NetworkResponder::SendData is called by the Network task, however when M552 S0 is executed by the Main task it loops through the network responder chain, terminating responders that are active and using that interface. The Terminate function in HttpResponder calls ConnectionLost, which releases the buffer chain and sets outbuf to null. The other types of responder may do likewise. So while the Network task is sending the data, the Main task may release the buffer.

A solution may be to have the Network task terminate the responders instead. The Network task already does the job of terminating the network connection, at least in the case of the WiFi interface.

It's likely that using M586 to repeatedly enable/disable HTTP protocol while a browser attempts to connect will cause a similar crash, because responders are also terminated when a protocol is disabled.

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Fixed in 3.5.3 and 3.6beta1.