Coverity analysis defect 13228: missing_lock: Accessing "this->_inNativeExecution" without holding lock

A new defect has been detected and assigned to redbeard0531 in Coverity Connect http://coverity.mongodb.com//sourcebrowser.htm?projectId=10001#mergedDefectId=10181
The defect was flagged by checker RESOURCE_LEAK in
file /src/mongo/shell/linenoise.cpp
function InputBuffer::incrementalHistorySearch(PromptBase &, int)
and this ticket was created by matt.kangas@10gen.com