Fix memory errors caused by using dangling pointers to SensorClients in SensorAgent
When some SensorClients are destroyed, eg. when KSysguard is closed, they fail to deregister themselves from SensorAgent. This may cause a crash when further messages arrive for them. This patch fixes that.
Note: this is an old patch from phabricator https://phabricator.kde.org/D29616 with changes requested there. But unfortunately wasn't applied