Windows: drive mapping enumeration infinite loop

If WNetEnumResource returns an error as opposed to success, such as
ERROR_UNEXP_NET_ERR, the enumeration loop would retry forever passing
zero for 'cEntries' which in turn results in a successful response
containing zero entries.

Change the while conditional to test for continued success instead
of ERROR_NO_MORE_ENTRIES.

Change-Id: I93af73b379aa455de7a8b3264b5178d482bb52b0
Reviewed-on: http://gerrit.openafs.org/9610
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>