Fix case when ANTLR getText() causes assertion in LexerInput.

[lkishalmi]

It seems getText() calls (only seen at error handling so far) from ANTLR could cause and assertion at LexerInput.readText()

This PR makes sure that we call the readText method with arguments that satisfy that condition.

[vieiro]

Hi @lkishalmi ,

I tried this with the current RustLexer (removing the EOF trick and doing some other adjustments) and I can see no more AssertionErrors now.

I'm getting some other errors [1], though. But maybe those are rust-lexer specific (I'll try to investigate during the weekend, maybe I can create a small grammar with some test case).

So a green light for me to merge this.

Thanks,
Antonio

[1]

java.lang.IllegalStateException: Lexer org.netbeans.modules.rust.grammar.RustLanguageLexer@4c0625c8
  returned null token but lexerInput.readLength()=13
  lexer-state: -1
  tokenStartOffset=3224, readOffset=3237, lookaheadOffset=3238
  Chars: "", x, z);\n}\n\n" - these characters need to be tokenized.
Fix the lexer to not return null token in this state.
	at org.netbeans.lib.lexer.LexerInputOperation.checkLexerInputFinished(LexerInputOperation.java:435)
	at org.netbeans.lib.lexer.LexerInputOperation.nextToken(LexerInputOperation.java:193)
	at org.netbeans.lib.lexer.inc.TokenListUpdater.relex(TokenListUpdater.java:602)
	at org.netbeans.lib.lexer.inc.TokenListUpdater.updateRegular(TokenListUpdater.java:255)
	at org.netbeans.lib.lexer.inc.TokenHierarchyUpdate$UpdateItem.update(TokenHierarchyUpdate.java:325)
	at org.netbeans.lib.lexer.inc.TokenHierarchyUpdate.processLevelInfos(TokenHierarchyUpdate.java:200)
	at org.netbeans.lib.lexer.inc.TokenHierarchyUpdate.updateImpl(TokenHierarchyUpdate.java:171)
	at org.netbeans.lib.lexer.inc.TokenHierarchyUpdate.update(TokenHierarchyUpdate.java:109)
	at org.netbeans.lib.lexer.TokenHierarchyOperation.textModified(TokenHierarchyOperation.java:585)
	at org.netbeans.spi.lexer.TokenHierarchyControl.textModified(TokenHierarchyControl.java:71)
	at org.netbeans.lib.lexer.inc.DocumentInput.textModified(DocumentInput.java:128)
	at org.netbeans.lib.lexer.inc.DocumentInput.insertUpdate(DocumentInput.java:117)
	at org.netbeans.lib.editor.util.swing.PriorityDocumentListenerList.insertUpdate(PriorityDocumentListenerList.java:60)
	at java.desktop/javax.swing.text.AbstractDocument.fireInsertUpdate(AbstractDocument.java:203)
	at org.netbeans.editor.BaseDocument.fireInsertUpdate(BaseDocument.java:1654)
	at org.netbeans.editor.BaseDocument.handleInsertString(BaseDocument.java:826)