Segfault in MergedIterator.pullTop()

[Adrien Grand]

FYI we have had 3 independent reports of segfaults in
org.apache.lucene.util.MergedIterator.pullTop() when running Elasticsearch
with Java 10. Here is one of them:
https://github.com/elastic/elasticsearch/issues/31425. It's still unclear
to me what conditions need to be met for this bug to reproduce. I'm
wondering whether anyone has seen this JVM bug before? For the record,
Elasticsearch doesn't use MergedIterator explicitly, it only gets used via
MultiFields when merging terms.