bp/XmlProcessor: copy caller_pool reference before destroying the XmlProcessor
bp/XmlProcessor: copy caller_pool reference before destroying the XmlProcessor

Fixes use-after-free bug caused by
872fd6f07fb22c96f342dbc178f36a620ea5d406

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy