PIG-3670: Fix assert in Pig script

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.12@1566077 13f79535-47bb-0310-9956-ffa450edef68
PIG-3347: Store invocation brings side effect

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/branch-0.12@1566082 13f79535-47bb-0310-9956-ffa450edef68