Fixed possible crash when classifying chars.

shared/cplusplus/Lexer.cpp

@@ -221,7 +221,7 @@ void Lexer::scan_helper(Token *tok)
         return;
     }
 
-    char ch = _yychar;
+    unsigned char ch = _yychar;
     yyinp();
 
     switch (ch) {

shared/cplusplus/Lexer.h

@@ -132,7 +132,7 @@ private:
     const char *_currentChar;
     const char *_lastChar;
     const char *_tokenStart;
-    char _yychar;
+    unsigned char _yychar;
     int _state;
     union {
         unsigned _flags;