Guard against checking if EOF is an invalid character.

Some encodings use a simple array lookup and EOF (-1) will always be an invalid index.

Author: nirvdrum

core/src/main/java/org/jruby/lexer/yacc/RubyLexer.java

@@ -1886,7 +1886,7 @@ private int identifier(int c, boolean commandState) throws IOException {
         do {
             if (!tokadd_mbchar(c)) return EOF;
             c = nextc();
-        } while (isIdentifierChar(c));
+        } while (c != EOF && isIdentifierChar(c));
 
         boolean lastBangOrPredicate = false;