Fix small leak in autosave.c getAutoDirName()

Another use of the recent routine gutils/fsys.c :: getFontForgeUserDir()
that requires a free() of the newly allocated string after use.

Tested and leak has disappeared from valgrind reports.
Merge pull request #1600 from tshinnic/leak_getuserdir_autosavec

Fix small leak in autosave.c getAutoDirName()