BUG/MINOR: proxy/session: Be sure to have a listener to increment its counters

BUG/MINOR: proxy/session: Be sure to have a listener to increment its counters

It is possible to have a session without a listener. It happens for applets
on the client side. Thus all accesses to the listener info from the session
must be guarded. It was the purpose of the commit 36119de18 ("BUG/MEDIUM:
session: NULL dereference possible when accessing the listener"). However,
some tests on the session's listener existence are missing in proxy_inc_*
functions.

This patch should fix the issues #1171, #1172, #1173, #1174 and #1175. It
must be backported with the above commit as far as 1.8.