Scary "read past EOF" in RAMDir [LUCENE-5849]

Nightly build hit this: http://builds.flonkings.com/job/Lucene-trunk-Linux-Java7-64-test-only/91095

And I'm able to repro at least once after beasting w/ the right JVM (1.7.0_55) and G1GC.

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Migrated from [LUCENE-5849](https://issues.apache.org/jira/browse/LUCENE-5849) by Michael McCandless (@mikemccand), updated Jul 30 2014
Attachments: [TestBinaryDocIndex.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5849/TestBinaryDocIndex.java)

Start with the linked nightly-build report and the attached TestBinaryDocIndex.java, then trace the RAMDir code involved in the reported read-past-EOF condition. Reproduce it with the stated Java 1.7.0_55 and G1GC environment if available. The issue does not define a specific expected fix or completion test, so the scope needs clarification before implementation.

Written by the indexing model from the issue text.