android: Prevent crash when trying to change pages in setup fragment

Sometimes when we want to change the current setup page, the current view isn't available and we try to alter the current view. This adds a guard to prevent that issue.

src/android/app/src/main/java/org/yuzu/yuzu_emu/fragments/SetupFragment.kt

@@ -355,11 +355,15 @@ class SetupFragment : Fragment() {
     }
 
     fun pageForward() {
-        binding.viewPager2.currentItem = binding.viewPager2.currentItem + 1
+        if (_binding != null) {
+            binding.viewPager2.currentItem += 1
+        }
     }
 
     fun pageBackward() {
-        binding.viewPager2.currentItem = binding.viewPager2.currentItem - 1
+        if (_binding != null) {
+            binding.viewPager2.currentItem -= 1
+        }
     }
 
     fun setPageWarned(page: Int) {