LanguageClient: fix crash in completion assist

Check m_processor again after starting. There is a callback resetting
the processor and we cannot ensure that starting the processor does not
result in calling this callback.

Change-Id: I4dca31399fb5224234c81df36edcb7b75d9ceaef
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/languageclient/languageclientcompletionassist.cpp

@@ -317,7 +317,7 @@ public:
         });
 
         setProposal(m_processor->start(std::move(interface)), prefix);
-        if (!m_processor->running()) {
+        if (m_processor && !m_processor->running()) {
             delete m_processor;
             m_processor = nullptr;
         }