Coverity analysis defect 13229: Accessing "mongo::dbexitCalled" without holding lock

A new defect has been detected and assigned to matt.kangas@10gen.com in Coverity Connect http://coverity.mongodb.com//sourcebrowser.htm?projectId=10001#mergedDefectId=13229
The defect was flagged by checker MISSING_LOCK in
file /src/mongo/shell/dbshell.cpp
function _main(int, char **, char **)
and this ticket was created by asya