pmxcfs: workaround dumb g_string_free behaviour

pmxcfs: workaround dumb g_string_free behaviour

While GLib mentions that this method is nullable[0][1] (i.e., can be passed
and can return null) its use of the, a bit misleading,
g_return_val_if_fail[2] voids that, as passing NULL emits an
warning[2] which looks pretty grave (assertion failure), albeit is
just noise..

[0]: https://developer.gnome.org/glib/stable/glib-Strings.html#g-string-free
[1]: https://salsa.debian.org/gnome-team/glib/blob/debian/2.58.3-2/glib/gstring.c#L196
[2]: https://developer.gnome.org/glib/stable/glib-Warnings-and-Assertions.html#g-return-val-if-fail

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>