Coverity analysis defect 24527: Dereference before null check

A new defect has been detected and assigned to milkie in Coverity Connect.
http://coverity.mongodb.com//sourcebrowser.htm?projectId=10001#mergedDefectId=24527
The defect was flagged by checker REVERSE_INULL in
file /src/mongo/db/d_concurrency.cpp
function mongo::Lock::TempRelease::TempRelease(mongo::LockState *)
and this ticket was created by milkie