Please re-run same test with the following patch, and see if the assertion is
failing :

diff --git a/src/mapgen/mapgen.c b/src/mapgen/mapgen.c
index bab7f38..abc7d67 100644
--- a/src/mapgen/mapgen.c
+++ b/src/mapgen/mapgen.c
@@ -467,6 +467,7 @@ void mapgen_convert(struct dungeon_info *di, int w, int h, u
 
        i = 0;
        while(tries && n) {
+               assert(i < di->num_rooms);
                if (idx[i] != di->enter && idx[i] != di->exit) {
                        if (make_corridor(idx[i]))
                                n--;