fix #K3890 - "wall_angle: unknown hwall mode 4"

When lastseentyp[x][y] is different from levl[x][y].typ and #terrain
tries to show <x,y>, the value of levl[x][y].flags might not have an
approrpriate value for the remembered terrain type.  The reported
problem was an impossible() about wall mode.

rm.wall_info == 4 corresponds to rm.doormask == D_OPEN and both of
them are overloaded on rm.flags.  A spot remembered as a wall but
actually a secret door might cause this if it has become discovered
('wall' changed to closed door) and then opened (with door intact)
while out of view.  [I'm not sure how that could happen though.]

I was unable to reproduce it so haven't verified that the fix works.

doc/fixes3-7-0.txt

@@ -1159,6 +1159,9 @@ change helm of brilliance from iron to crystal so that it no longer needs to
 fake player monsters in endgame had a 1 in 1000 chance to be given a stack
 	of 0 gold pieces, eventually triggering an impossible warning (cited
 	one was "Calculating weight of 0 gold pieces?")
+avoid "wall_angle: unknown hwall mode 4" when using #terrain command to
+	display a spot remembered as a wall but actually a secret door that
+	has been transformed into an open door while not in sight
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository