Issue 7835: Shelve's __del__ method calls its close method, and its

Issue 7835:  Shelve's __del__ method calls its close method, and its
close method refers to an identifier in the global module namespace.
This means that when __del__ is called during interpreter shutdown (if,
for example, the calling program still has a pointer to the shelf),
sometimes that global identifier would wind up being None, causing
mysterious 'ignored' exceptions.  This patch checks for the possible None
value first before using the global, thus avoiding the error messages.