Commit 2027223a created a panic when attempting to clear the dirlist
when the config option dirlist-tree is set to false.
This patch fixes that panic by creating a dirlist.ClearList() function
to prevent needing to check a callback.
Tested with both dirlist-tree=false and true
Fixes: 2027223ab3 ("fix: clear dirlist on disconnect")
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>
115447e57f