[BUG] client: always ensure to zero rep->analysers

[BUG] client: always ensure to zero rep->analysers

The response analyser was not emptied upon creation of a new session. In
fact it was always zero just because last session leaved it in a zero state,
but in case of shared pools this cannot be guaranteed. The net effect is
that it was possible to have some HTTP (or any other) analysers on the
response path of a stats unix socket, which would reject the response.

This fix must be backported to 1.4.