Segfault / sentinel

[dirkx]

During a congested network - I got reproducable segaults in:

https://github.com/me-no-dev/ESPAsyncWebServer/blob/67de9cddbbf0d024c61a314bacd204ba6101fac8/src/AsyncWebSocket.cpp#L646

it appears that data can be NULL (and datalen = 0). Given that in line

https://github.com/me-no-dev/ESPAsyncWebServer/blob/67de9cddbbf0d024c61a314bacd204ba6101fac8/src/AsyncWebSocket.cpp#L698

this is only used when datalen >0 I suggest putting an

 uint8_t datalast;
 if (data && datalen>0)  datalast = data[datalen];

sentinel in.