Fixed a few unchecked nulls on RAMCloudTransaction.read().
Fixed a few unchecked nulls on RAMCloudTransaction.read().

Found a few places where I was not checking for null return values from
RAMCloudTransaction.read().

ellitron/ramcloud-gremlin,jdellithorpe/torc-gremlin,jdellithorpe/torc-gremlin,ellitron/ramcloud-gremlin