Guard websocket client against invalidation of TCP socket

Based on commit c9e4d42 of dumbfixes
branch of 0xFEEDC0DE64 fork of ESPAsyncWebServer.

When the socket gets disconnected, its corresponding AsyncClient is set
to NULL. Several places in the code now have null pointer checks to
prevent crashes.
Message queue overflow is now an error that triggers socket
disconnection. Limit of queued messages is now 16 for ESP32.
Disconnected clients are no longer immediately removed from the client
queue. Instead the cleanupClients() function prunes all of the
disconnected clients.
Some code formatting fixes.

Author: avillacis