webbing over stairs

Due to limitations in some interface's display capabilities, don't let
polymorphed players web over stairs or ladders.  As a side effect, this
side-steps missing checks for webs when going up or down stairs and ladders.

doc/fixes34.2

@@ -93,6 +93,7 @@ options for font_size for map, menu, message, status, and text all had the
 when dismounting by choice and unimpaired, try not to land in a known trap
 when jousting a pudding into a polymorh trap, it was possible to end up
 	with two of the new type of monster
+don't allow polymorphed player to web over the stairs
 
 
 Platform- and/or Interface-Specific Fixes

src/polyself.c

@@ -891,6 +891,13 @@ dospinweb()
 		default:
 			impossible("Webbing over trap type %d?", ttmp->ttyp);
 			return(0);
+		}
+	else if (On_stairs(u.ux, u.uy)) {
+	    /* cop out: don't let them hide the stairs */
+	    Your("web fails to impede access to the %s.",
+		 (levl[u.ux][u.uy].typ == STAIRS) ? "stairs" : "ladder");
+	    return(1);
+		 
 	}
 	ttmp = maketrap(u.ux, u.uy, WEB);
 	if (ttmp) {