[patch] Fix a couple of null-derefs in setup.exe

[Dave Korn]

    Hi gang,

  There are a couple of places where setup can bomb if you blow away your
stored settings (cached mirror list or last mirror), as it gets a null pointer
on trying to read them back; both strtok and the std::string(const char *)
ctor blow up on this.  Attached patch trivially protects against both.

	* site.cc (get_site_list): Use empty string if user settings
	return NULL pointer for cached mirrors list.
	(SiteSetting::getSavedSites): Do nothing if user settings return
	NULL pointer for last mirror URL.

  Ok?

    cheers,
      DaveK
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