QmlDebug: Fix 'Interrupt' functionality for mixed debugging

Make sure that m_inferiorPid is initialized even when the CppEngine
is not the main one, and therefore startDebugger() not called on it.
Otherwise the engine tries to send a SIGINT signal to an arbirary
process ID, resulting in the debugging being stopped.

Reviewed-by: Friedemann Kleint
Task-number: QTCREATORBUG-4716

src/plugins/debugger/debuggerengine.cpp

@@ -168,6 +168,7 @@ public:
         m_state(DebuggerNotReady),
         m_lastGoodState(DebuggerNotReady),
         m_targetState(DebuggerNotReady),
+        m_inferiorPid(0),
         m_modulesHandler(),
         m_registerHandler(),
         m_sourceFilesHandler(),