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/codeassistant.cpp

@@ -288,6 +288,8 @@ void CodeAssistantPrivate::cancelCurrentRequest()
         m_requestRunner->setDiscardProposal(true);
         disconnect(m_runnerConnection);
     }
+    if (m_asyncProcessor)
+        m_asyncProcessor->cancel();
     invalidateCurrentRequestData();
 }