TestRandomChains.testRandomChainsWithLargeStrings() failures [LUCENE-7805]

My Jenkins found a reproducing master seed, looks like FlattenGraphFilter is where the problem happens:

```
Checking out Revision 680f4d7fd378868254786107de92a894758f667c (refs/remotes/origin/master)
[...]
[junit4] Suite: org.apache.lucene.analysis.core.TestRandomChains
[junit4] 2> TEST FAIL: useCharFilter=false text='\u0003J\u522f nwqbl uwtps ob zdyokom ){0'
[junit4] 2> Exception from random analyzer:
[junit4] 2> charfilters=
[junit4] 2> org.apache.lucene.analysis.charfilter.HTMLStripCharFilter(java.io.StringReader@3ab617ae, [])
[junit4] 2> org.apache.lucene.analysis.charfilter.HTMLStripCharFilter(org.apache.lucene.analysis.charfilter.HTMLStripCharFilter@23e3c717)
[junit4] 2> tokenizer=
[junit4] 2> org.apache.lucene.analysis.ngram.NGramTokenizer(9, 43)
[junit4] 2> filters=
[junit4] 2> org.apache.lucene.analysis.miscellaneous.CodepointCountFilter(ValidatingTokenFilter@6b4708ea term=,bytes=[],startOffset=0,endOffset=0,positionIncrement=1,positionLength=1,type=word, 33, 44)
[junit4] 2> org.apache.lucene.analysis.shingle.ShingleFilter(ValidatingTokenFilter@5533fb25 term=,bytes=[],startOffset=0,endOffset=0,positionIncrement=1,positionLength=1,type=word, )
[junit4] 2> org.apache.lucene.analysis.core.FlattenGraphFilter(ValidatingTokenFilter@4ef4c44 term=,bytes=[],startOffset=0,endOffset=0,positionIncrement=1,positionLength=1,type=word)
[junit4] 2> org.apache.lucene.analysis.miscellaneous.KeepWordFilter(ValidatingTokenFilter@15baa1c7 term=,bytes=[],startOffset=0,endOffset=0,positionIncrement=1,positionLength=1,type=word, [akbnucwt, vrkwm, jtomhk, jxgmfalr])
[junit4] 2> offsetsAreCorrect=true
[junit4] 2> NOTE: reproduce with: ant test -Dtestcase=TestRandomChains -Dtests.method=testRandomChainsWithLargeStrings -Dtests.seed=E9460213902F2F82 -Dtests.slow=true -Dtests.locale=fi-FI -Dtests.timezone=Europe/Malta -Dtests.asserts=true -Dtests.file.encoding=US-ASCII
[junit4] FAILURE 0.19s J7 | TestRandomChains.testRandomChainsWithLargeStrings <<<
[junit4] > Throwable #1: java.lang.AssertionError: outputEndNode=3 vs inputTo=2
[junit4] > at __randomizedtesting.SeedInfo.seed([E9460213902F2F82:831DBD02C9610F71]:0)
[junit4] > at org.apache.lucene.analysis.core.FlattenGraphFilter.incrementToken(FlattenGraphFilter.java:335)
[junit4] > at org.apache.lucene.analysis.ValidatingTokenFilter.incrementToken(ValidatingTokenFilter.java:67)
[junit4] > at org.apache.lucene.analysis.FilteringTokenFilter.incrementToken(FilteringTokenFilter.java:51)
[junit4] > at org.apache.lucene.analysis.ValidatingTokenFilter.incrementToken(ValidatingTokenFilter.java:67)
[junit4] > at org.apache.lucene.analysis.BaseTokenStreamTestCase.checkAnalysisConsistency(BaseTokenStreamTestCase.java:731)
[junit4] > at org.apache.lucene.analysis.BaseTokenStreamTestCase.checkRandomData(BaseTokenStreamTestCase.java:642)
[junit4] > at org.apache.lucene.analysis.BaseTokenStreamTestCase.checkRandomData(BaseTokenStreamTestCase.java:540)
[junit4] > at org.apache.lucene.analysis.core.TestRandomChains.testRandomChainsWithLargeStrings(TestRandomChains.java:880)
[junit4] > at java.lang.Thread.run(Thread.java:745)
[junit4] 2> NOTE: test params are: codec=Asserting(Lucene70): {dummy=PostingsFormat(name=LuceneVarGapFixedInterval)}, docValues:{}, maxPointsInLeafNode=807, maxMBSortInHeap=5.007333045299232, sim=RandomSimilarity(queryNorm=true): {}, locale=fi-FI, timezone=Europe/Malta
[junit4] 2> NOTE: Linux 4.1.0-custom2-amd64 amd64/Oracle Corporation 1.8.0_77 (64-bit)/cpus=16,threads=1,free=492062472,total=525336576
```

---
Migrated from [LUCENE-7805](https://issues.apache.org/jira/browse/LUCENE-7805) by Steven Rowe (@sarowe), updated May 24 2018

Start by running the reproduced ant command for TestRandomChains.testRandomChainsWithLargeStrings using seed E9460213902F2F82. Read TestRandomChains.java around line 880 and FlattenGraphFilter.java around line 335, then trace the outputEndNode/inputTo assertion. Done means the reproducer passes without the assertion failure.

Written by the indexing model from the issue text.