Android: only send Quit event to SDLThread if it's not already terminated

And it avoids reporting errors using Android_Pause/ResumeSem that are NULL.

android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -390,16 +390,18 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
            return;
         }
 
-        // Send a quit message to the application
         SDLActivity.mExitCalledFromJava = true;
-        SDLActivity.nativeQuit();
 
-        // Now wait for the SDL thread to quit
         if (SDLActivity.mSDLThread != null) {
+
+            // Send Quit event to "SDLThread" thread
+            SDLActivity.nativeSendQuit();
+
+            // Wait for "SDLThread" thread to end
             try {
                 SDLActivity.mSDLThread.join();
             } catch(Exception e) {
-                Log.v(TAG, "Problem stopping thread: " + e);
+                Log.v(TAG, "Problem stopping SDLThread: " + e);
             }
         }
 
@@ -724,7 +726,7 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
     public static native int nativeSetupJNI();
     public static native int nativeRunMain(String library, String function, Object arguments);
     public static native void nativeLowMemory();
-    public static native void nativeQuit();
+    public static native void nativeSendQuit();
     public static native void nativePause();
     public static native void nativeResume();
     public static native void onNativeDropFile(String filename);

src/core/android/SDL_android.c

@@ -122,7 +122,7 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(onNativeClipboardChanged)(
 JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeLowMemory)(
         JNIEnv *env, jclass cls);
 
-JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeQuit)(
+JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeSendQuit)(
         JNIEnv *env, jclass cls);
 
 JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativePause)(
@@ -813,12 +813,12 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeLowMemory)(
     SDL_SendAppEvent(SDL_APP_LOWMEMORY);
 }
 
-/* Quit */
-JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeQuit)(
+/* Send Quit event to "SDLThread" thread */
+JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeSendQuit)(
                                     JNIEnv *env, jclass cls)
 {
     /* Discard previous events. The user should have handled state storage
-     * in SDL_APP_WILLENTERBACKGROUND. After nativeQuit() is called, no
+     * in SDL_APP_WILLENTERBACKGROUND. After nativeSendQuit() is called, no
      * events other than SDL_QUIT and SDL_APP_TERMINATING should fire */
     SDL_FlushEvents(SDL_FIRSTEVENT, SDL_LASTEVENT);
     /* Inject a SDL_QUIT event */