StringIndexOutOfBoundsException in parser/EscapeQuerySyntaxImpl.java [LUCENE-8572]

With "lucene-queryparser-6.3.0", specifically in
"org/apache/lucene/queryparser/flexible/standard/parser/EscapeQuerySyntaxImpl.java"
 
when escaping strings containing extended unicode chars, and with a locale distinct from that of the character set the string uses, the process fails, with a "java.lang.StringIndexOutOfBoundsException".
 
The reason is that the comparison is done by previously converting all of the characters of the string to lower case chars, and by doing this, the original string size isn't anymore the same, but less, as of the transformed one, so that executing
 
org/apache/lucene/queryparser/flexible/standard/parser/EscapeQuerySyntaxImpl.java:89

fails with a java.lang.StringIndexOutOfBoundsException.

I wonder whether the transformation to lower case is really needed when treating the escape chars, since by avoiding it, the error may be avoided.

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Migrated from [LUCENE-8572](https://issues.apache.org/jira/browse/LUCENE-8572) by Octavian Mocanu (@tonicava), updated Aug 10 2019

Start with org/apache/lucene/queryparser/flexible/standard/parser/EscapeQuerySyntaxImpl.java, especially line 89, and reproduce the failure using lucene-queryparser-6.3.0 with extended Unicode characters under a differing locale. Trace the lower-case transformation and verify that escaping such strings no longer raises StringIndexOutOfBoundsException.

Written by the indexing model from the issue text.