For the case when the connection struct is all setup, the protocol
handler allocates data in its setup_connection function, but the
connection struct is discarded again before used further because a
connection reuse is prefered. Then the handler's disconnect function was
not previously called, which then would lead to a memory leak.
I added test case 698 that reproduces the leak and the fix.
Reported-by: Philippe Antoine
Closes#16604
