Fixes#5701 WDTs and other issues with BearSSL::WiFiServerSecure
The BSSL server was creating the client it returns on a connection in a
way that caused the counter for the stack_thunk to get out of sync and
cause it to be freed improperly by having the destructor be called one
more time than the constructor. Looks like RVO.
Rewrite the ::available() function in order to avoid this issue with
help from @devyte.