fix Coverity issues

src/App/Expression.h

@@ -507,7 +507,7 @@ public:
   std::string string;
   FunctionExpression::Function func;
   ObjectIdentifier::String string_or_identifier;
-  semantic_type() {}
+  semantic_type() : expr(0), ivalue(0), fvalue(0), func(FunctionExpression::NONE) {}
 };
 
 #define YYSTYPE semantic_type