Debugger: Avoid soft assert on close

Introduced with d432b14472.

Change-Id: Ic33d46a33efd1c09c162c6adc31526efd4fe2b9d
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/debugger/debuggerplugin.cpp

@@ -2759,9 +2759,11 @@ void DebuggerPluginPrivate::coreShutdown()
 {
     m_shuttingDown = true;
     if (currentEngine()) {
+        if (currentEngine()->state() != Debugger::DebuggerNotReady) {
             currentEngine()->setTargetState(Debugger::DebuggerFinished);
             currentEngine()->abortDebugger();
         }
+    }
 }
 
 const CPlusPlus::Snapshot &cppCodeModelSnapshot()