fix bug: missing initialization for lexer context variable

src/macro/MacroFlex.ll

@@ -422,6 +422,7 @@ CONT \\\\
 
 MacroFlex::MacroFlex(istream* in, ostream* out, bool no_line_macro_arg, vector<string> path_arg)
   : MacroFlexLexer(in, out), input(in), no_line_macro(no_line_macro_arg), path(path_arg),
+    is_for_context(false),
     reading_for_statement(false), reading_if_statement(false)
 {
 }