Fix a crash when using the back button in a search results list with "Manage Back button" preference enabled.

Fixes issue 1716

src/com/fsck/k9/activity/MessageList.java

@@ -581,10 +581,17 @@ public class MessageList
         // or later, or by the code above on earlier versions of the
         // platform.
         if (K9.manageBack())
+        {
+            if (mQueryString == null)
             {
                 onShowFolderList();
             }
             else
+            {
+                onAccounts();
+            }
+        }
+        else
         {
             finish();
         }