Unbreak ! on portal vault exits and escape hatches up.

level_id::get_next_level_id() checks whether the stair feature is a
branch exit for some branch; if so, it returns that branch's parent.
However, this is wrong for:
 * portal exits, where there is no parent;
 * dungeon exits, for the same reason;
 * escape hatches up, which are sometimes but not usually Lab exits.

However, portal escape hatches up are branch exits (from Lab), and Lab
does not have a parent.

Instead, check only for the current branch's exit; if the feature does
match, find the destination using the same code as up_stairs().  We
split this code off into a new (global) function upstairs_destination().
For exits from D:1, return D:0 rather than NUM_BRANCHES:-1.

1 files changed, 1 insertions, 0 deletions

diff --git a/crawl-ref/source/stairs.h b/crawl-ref/source/stairs.h
index b1bce5336c..36fad4bc0c 100644
--- a/crawl-ref/source/stairs.h
+++ b/crawl-ref/source/stairs.h
@@ -7,6 +7,7 @@
 #define STAIRS_H
 
 bool check_annotation_exclusion_warning();
+level_id upstairs_destination();
 void down_stairs(dungeon_feature_type force_stair = DNGN_UNSEEN);
 void up_stairs(dungeon_feature_type force_stair = DNGN_UNSEEN);
 void new_level(bool restore = false);