Fix for an exception when a non glusterfs URI is passed into the sameVolume code which has no scheme

If the mapreduce.jobtracker.system.dir property is undefined, then mapred-site-default.xml is loaded from JAR which set the value to /tmp/hadop/something

this value is eventually checked in sameVolume, and since there is no scheme the check throws an NPE.  this pull is a fix for that.