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)
1 file changed