Adapt to g_string_free glib 2.75.3 change

glib 2.75.3 changes warning behaviour of `g_string_free()`.
See:
* https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3219
* https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3226

Use this opportunity to replace the use of GString with
`g_strdup_printf()` where possible.
Otherwise correctly take the return value of `g_string_free()`
which is nicer anyways.

src/command/cmd_defs.c

@@ -2738,10 +2738,7 @@ _cmd_index(const Command* cmd)
     }
     g_strfreev(tokens);
 
-    char* res = index->str;
-    g_string_free(index, FALSE);
-
-    return res;
+    return g_string_free(index, FALSE);
 }
 
 GList*