prevent a panic splitbill: no resident shopkeeper

Patch was sent in by <Someone> on Sep 8:
This bug causes a number of impossible messages (starting with splitbill:
no resident shopkeeper??)

Repeat by:

Enter a large shop.
Wish for a large stack of projectiles.
Sell your projectiles and then pick them up again.
Trap shopkeeper against the door with a scroll of earth.
Throw the projectiles at the shopkeeper to anger him.
Move away from the boulder trap and wait for the shopkeeper to leave the
shop.
Throw one of the projectiles at the free space.

doc/fixes34.4

@@ -54,6 +54,7 @@ ensure that a sleeping steed doesn't answer a #chat
 eliminate two very minor anomalies when using Luck to adjust random numbers
 destroying a worn item via dipping in burning oil would not unwear/unwield
 	the item properly, possibly leading to various strange behaviors
+avoid a panic splitbill when shopkeeper is trapped by the door
 
 
 Platform- and/or Interface-Specific Fixes

src/shk.c

@@ -3228,7 +3228,7 @@ register struct monst *shkp;
 		    Such voluntary abandonment left unpaid objects in
 		    invent, triggering billing impossibilities on the
 		    next level once the character fell through the hole.] */
-		if (udist > 4 && eshkp->following)
+		if (udist > 4 && eshkp->following && !eshkp->billct)
 		    return(-1);	/* leave it to m_move */
 		gx = u.ux;
 		gy = u.uy;