StringIndexOutOfBoundsException in FastVectorHighlighter [LUCENE-9591]

Just spotted this one. Solr throws the exception but the request is returned anyway. One highlighted snippet has garbled contents.

```java
2020-10-29 15:06:30.448 ERROR (qtp2119992687-25889) [c:collection_20200511 s:shard4 r:core_node48 x:sitesearch_20200511_shard4_replica_t47] o.a.s.s.HttpSolrCall null:java.lang.StringIndexOutOfBoundsException: start 568858, end 539731, length 539732

       at java.base/java.lang.AbstractStringBuilder.checkRangeSIOOBE(AbstractStringBuilder.java:1724)

       at java.base/java.lang.AbstractStringBuilder.substring(AbstractStringBuilder.java:1017)

       at java.base/java.lang.StringBuilder.substring(StringBuilder.java:85)

       at org.apache.lucene.search.vectorhighlight.BaseFragmentsBuilder.getFragmentSourceMSO(BaseFragmentsBuilder.java:204)

       at org.apache.lucene.search.vectorhighlight.BaseFragmentsBuilder.makeFragment(BaseFragmentsBuilder.java:175)

       at org.apache.lucene.search.vectorhighlight.BaseFragmentsBuilder.createFragments(BaseFragmentsBuilder.java:144)

       at org.apache.lucene.search.vectorhighlight.FastVectorHighlighter.getBestFragments(FastVectorHighlighter.java:186)

       at org.apache.solr.highlight.DefaultSolrHighlighter.doHighlightingByFastVectorHighlighter(DefaultSolrHighlighter.java:581)

       at org.apache.solr.highlight.DefaultSolrHighlighter.doHighlightingOfField(DefaultSolrHighlighter.java:531)

       at org.apache.solr.highlight.DefaultSolrHighlighter.doHighlighting(DefaultSolrHighlighter.java:478)

       at org.apache.solr.handler.component.HighlightComponent.process(HighlightComponent.java:172)

```

---
Migrated from [LUCENE-9591](https://issues.apache.org/jira/browse/LUCENE-9591) by Markus Jelsma
Linked issues:
- #8073

Start by reading BaseFragmentsBuilder.getFragmentSourceMSO at the stack-trace location, then follow how FastVectorHighlighter reaches it and review linked issue #8073. Reproduce the reported request if possible; done means the StringIndexOutOfBoundsException no longer occurs and the highlighted snippet is not garbled.

Written by the indexing model from the issue text.