Add a null pointer check in NativeActivity

due to stack trace from #17364

Author: hrydgard

android/src/org/ppsspp/ppsspp/NativeActivity.java

@@ -754,7 +754,7 @@ protected void onDestroy() {
 		super.onDestroy();
 		Log.i(TAG, "onDestroy");
 		if (javaGL) {
-			if (nativeRenderer.isRenderingFrame()) {
+			if (nativeRenderer != null && nativeRenderer.isRenderingFrame()) {
 				Log.i(TAG, "Waiting for renderer to finish.");
 				int tries = 200;
 				do {