fc-cache segfaults within FcDirScan() because FcDirScan()
does:
       if (FcDirCacheReadDir (set, dirs, dir))
       {
          FcGlobalCacheReferenceSubdir (cache, dir);
          return FcTrue;
        }

But cache can be NULL. Adding a if (cache) before
calling ReferenceSubdir() looks right and tests to fix
the problem.

[ Reproduction:

  touch /usr/share/fonts/ja/Default
  touch /usr/share/fonts/ja
  touch /usr/share/fonts
  fc-list
  fc-cache /usr/share/fonts/ja

  Probably can be made a lot more minimal, but the fix was obvious
 ]