Fix crash when finishing IndividualPickerActivity

Calling finish() doesn't leave the current method, so as we
didn't return immediately, we were continuing with a null
mCategory

Bug: 129006832
Change-Id: Ib26acfee7cb6657eaf0823ac594b017c744b7341
diff --git a/src/com/android/wallpaper/picker/individual/IndividualPickerActivity.java b/src/com/android/wallpaper/picker/individual/IndividualPickerActivity.java
index 24d81ea..2d77608 100755
--- a/src/com/android/wallpaper/picker/individual/IndividualPickerActivity.java
+++ b/src/com/android/wallpaper/picker/individual/IndividualPickerActivity.java
@@ -95,6 +95,7 @@
             // saved state, or with a collection id that doesn't exist anymore.
             // In those cases, we cannot continue, so let's just go back.
             finish();
+            return;
         }
 
         setTitle(mCategory.getTitle());