SyncSourceFeedback::replHandshake() may perform an illegal erase from a std::map in some circumstances

SyncSourceFeedback::replHandshake() calls _slaveMap.erase(_slaveMap.find(removeItr->first)). However, _slaveMap.find(removeItr->first) may return _slaveMap.end(), which is an illegal argument to _slaveMap.erase(). This could lead to memory corruption.