Fix memory leak

As documented in https://github.com/karlseguin/ccache/issues/76, an entry which
is both GC'd and deleted (either via a delete or an update) will result in the
internal link list having a nil tail (because removing the same node multiple times
from the linked list does that).

doDelete was already aware of "invalid" nodes (where item.node == nil), so the
solution seems to be as simple as setting item.node = nil during GC.

cache.go

@@ -438,6 +438,7 @@ func (c *Cache[T]) gc() int {
 				c.onDelete(item)
 			}
 			dropped += 1
+			item.node = nil
 			item.promotions = -2
 		}
 		node = prev