macos: do not quit prefpane unexpectedly

macos: do not quit prefpane unexpectedly

If the user opens the OpenAFS preference pane and choose the Mounts
tab, the preference pane crashes.

To fix the problem, do not assume that we can cast a NSdictionary
object to NSMutableDictionary.

Reviewed-on: https://gerrit.openafs.org/12446
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 9d4be0bd01696768602a313f627a802b358b5885)

Change-Id: I7ff8c6cbc599f3e80d6365d9a56587bf5c641f5b
Reviewed-on: https://gerrit.openafs.org/12447
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Joe Gorse <jhgorse@gmail.com>
Tested-by: Joe Gorse <jhgorse@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>