Android: fix IllegalStateException in onBackPressed()

Rare exception, not catch-able, which appears when the activity gets in a broken
state.

java.lang.IllegalStateException:
  at android.app.FragmentManagerImpl.checkStateLoss (FragmentManagerImpl.java:1323)
  at android.app.FragmentManagerImpl.popBackStackImmediate (FragmentManagerImpl.java:493)
  at android.app.Activity.onBackPressed (Activity.java:2215)
  at org.libsdl.app.SDLActivity.onBackPressed (SDLActivity.java)
  at android.app.Activity.onKeyUp (Activity.java:2193)
  at android.view.KeyEvent.dispatch (KeyEvent.java:2685)
  at android.app.Activity.dispatchKeyEvent (Activity.java:2423)
  at org.libsdl.app.SDLActivity.dispatchKeyEvent (SDLActivity.java)

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

@@ -453,7 +453,9 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
         }
 
         // Default system back button behavior.
-        super.onBackPressed();
+        if (!isFinishing()) {
+            super.onBackPressed();
+        }
     }
 
     // Called by JNI from SDL.
@@ -466,7 +468,9 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
         runOnUiThread(new Runnable() {
             @Override
             public void run() {
-                SDLActivity.this.superOnBackPressed();
+                if (!SDLActivity.this.isFinishing()) {
+                    SDLActivity.this.superOnBackPressed();
+                }
             }
         });
     }