Coverity analysis defect 15708: update.cpp, Dereference after null check

A new defect has been detected and assigned to acm in Coverity Connect http://coverity.mongodb.com//sourcebrowser.htm?projectId=10001#mergedDefectId=15708
The defect was flagged by checker FORWARD_NULL in
file /src/mongo/db/ops/update.cpp
function mongo::update(const mongo::UpdateRequest &, mongo::OpDebug *, mongo::UpdateDriver *)
and this ticket was created by matt.kangas@10gen.com