Carla: Cleanup standalone code, misc fixing

c++/carla-backend/carla_backend_standalone.h

@@ -44,7 +44,7 @@ struct PluginInfo {
     PluginInfo()
         : type(CarlaBackend::PLUGIN_NONE),
           category(CarlaBackend::PLUGIN_CATEGORY_NONE),
-          hints(0),
+          hints(0x0),
           binary(nullptr),
           name(nullptr),
           label(nullptr),

c++/carla-backend/carla_osc.cpp

@@ -22,7 +22,7 @@ CARLA_BACKEND_START_NAMESPACE
 
 void osc_error_handler(const int num, const char* const msg, const char* const path)
 {
-    qCritical("osc_error_handler(%i, %s, %s)", num, msg, path);
+    qCritical("CarlaBackend::osc_error_handler(%i, \"%s\", \"%s\")", num, msg, path);
 }
 
 CarlaOsc::CarlaOsc(CarlaEngine* const engine_)

c++/carla-backend/carla_threads.cpp

@@ -44,6 +44,9 @@ void CarlaCheckThread::startNow()
 
 void CarlaCheckThread::stopNow()
 {
+    if (m_stopNow)
+        return;
+
     m_stopNow = true;
 
     // TESTING - let processing finish first