Fix crash using dangling (const!) iterator after remove in Conference code
Fix crash using dangling (const!) iterator after remove in Conference code

Reviewed-by: Will Thompson (wjt) <5822fdda638242a44ccfa68f146ccf6f257c5cbd@collabora.co.uk>

TelepathyQt/telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt,anantkamath/telepathy-qt,tiagosh/telepathy-qt,tiagosh/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyQt/telepathy-qt,special/telepathy-qt-upstream,detrout/telepathy-qt,anantkamath/telepathy-qt,detrout/telepathy-qt,anantkamath/telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt4,TelepathyIM/telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt4,freedesktop-unofficial-mirror/telepathy__telepathy-qt,TelepathyIM/telepathy-qt,special/telepathy-qt-upstream,TelepathyQt/telepathy-qt,tiagosh/telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt,tiagosh/telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt4,special/telepathy-qt-upstream,TelepathyIM/telepathy-qt,TelepathyIM/telepathy-qt,freedesktop-unofficial-mirror/telepathy__telepathy-qt4,TelepathyQt/telepathy-qt,detrout/telepathy-qt