Cancel session request if the task is deleted early.

E.g. due to losing the connection to the server.
Otherwise the request is processed later on, with no task to use that
session, and we end up with a session in m_reservedSession for ever, and
soon afterwards an infinite stream of
"Cancelling this request. Probably there is no more session available."

Might be related to:
CCBUG: 318098, 316541
(but for lack of debug output in these reports, I can't tell for sure)

FIXED-IN: 4.10.5
Reviewed-by: Kévin Ottens