[FLINK-2689] [runtime] Fix reuse of null object for solution set Joins and CoGroups

[fhueske]

Reuse object can become null if not match is found in the hash table. The next look-up will happen with a null-valued reuse object and fail.

This patch takes care that the reuse object does not become null.

[mxm]

LGTM

[fhueske]

Thanks for the review.
Will merge this.

[srowen]

@fhueske not sure where else to mention this, but for some reason, PRs like this are causing Apache JIRA to comment on Spark JIRAs (see https://issues.apache.org/jira/browse/SPARK-2689 for instance). Is it maybe because the merge script or other setup from Spark is also used in Flink? Maybe something wasn't changed. Not a huge deal but worth tracking down

[rmetzger]

Hi @srowen, thank you for notifying us. We were trying out https://github.com/databricks/spark-pr-dashboard on Google App Engine (because we are also getting more and more flooded by pull requests ;) ). It seems that we didn't properly configure and test the tool before starting it.
After you told us about the issue, we immediately stopped the tool. You can also see from Max (@mxm)'s JIRA activity (https://issues.apache.org/jira/secure/ViewProfile.jspa?name=mxm) that he stopped changing Spark JIRAs three hours ago.
I'm really sorry that this has happened.
We'll manually clean up the Spark JIRAs.