fix crash when deleting nested default deck

https://anki.tenderapp.com/discussions/ankidesktop/25777-bug-report-syn

anki/decks.py

@@ -153,8 +153,16 @@ class DeckManager:
             # child of an existing deck then it needs to be renamed
             deck = self.get(did)
             if '::' in deck['name']:
-                deck['name'] = _("Default")
-                self.save(deck)
+                base = deck['name'].split("::")[-1]
+                suffix = ""
+                while True:
+                    # find an unused name
+                    name = base + suffix
+                    if not self.byName(name):
+                        deck['name'] = name
+                        self.save(deck)
+                        break
+                    suffix += "1"
             return
         # log the removal regardless of whether we have the deck or not
         self.col._logRem([did], REM_DECK)