Fix small leak in pythonui.c python_ui_setup_callback()

Another use of the recent routine `gutils/fsys.c` :: `getFontForgeUserDir()`
that requires a `free()` of the newly allocated string after use.

Note that there was an unprotected path if a NULL was returned from
`getFontForgeUserDir()`.  The old code would have tried to use the null
pointer.  The fix protects against that, but rather does 'nothing' if
NULL is returned.  Maybe it should blow up *horrible bad* instead?