shop credit fix

shop credit fix

[ Fix a bug reported in the newsgroup; I thought I sent this last
  week, but it isn't in the current code so I must have forgotten. ]

     When I split u_left_shop() into two routines I neglected to
propagate the early return condition from the second half to the
first.  The result is that if you leave a shop with unpaid goods
but have enough credit there to cover the cost, the shopkeeper
will take that credit and be satisified, but the kops were still
getting summoned as if he had been robbed.