core: Reset cpu_core after kernel.

- Fixes a crash on Android.

core.cpp: check if video_dumper is created before trying to shut it down

src/core/core.cpp

@@ -541,11 +541,11 @@ void System::Shutdown(bool is_deserializing) {
     archive_manager.reset();
     service_manager.reset();
     dsp_core.reset();
-    cpu_cores.clear();
     kernel.reset();
+    cpu_cores.clear();
     timing.reset();
 
-    if (video_dumper->IsDumping()) {
+    if (video_dumper && video_dumper->IsDumping()) {
         video_dumper->StopDumping();
     }