PIG-3469: Skewed join can cause unrecoverable NullPointerException when one of its inputs is missing (Jarek Jarcec Cecho  via xuefuz)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1529011 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk