channel-text.c: on_received_signal_cb: don't unref @log if it's uninitialized
channel-text.c: on_received_signal_cb: don't unref @log if it's uninitialized

freedesktop-unofficial-mirror/telepathy__telepathy-logger,freedesktop-unofficial-mirror/telepathy__telepathy-logger,freedesktop-unofficial-mirror/telepathy__telepathy-logger