C++: initialize class field.

Change-Id: I3bcc08356951518d9c7c3ae7974b475370112dfe
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/libs/3rdparty/cplusplus/AST.h

@@ -4467,6 +4467,7 @@ public:
         , mutable_token(0)
         , exception_specification(0)
         , trailing_return_type(0)
+        , symbol(0)
     {}
 
     virtual LambdaDeclaratorAST *asLambdaDeclarator() { return this; }