CodeAssistant: add cancel to asynchronous processors

Fixes crash when the processor reports a result after the code assistant
was destroyed.

Change-Id: I8588d3d6acad69f1ec6302e8ba09d642ebbb77f1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/codeassist/iassistprocessor.h

@@ -51,6 +51,7 @@ public:
 
     virtual bool running() { return false; }
     virtual bool needsRestart() const { return false; }
+    virtual void cancel() {}
 
 private:
     AsyncCompletionsAvailableHandler m_asyncCompletionsAvailableHandler;