Don't pay the shopkeeper when displacing a pet in a shop

The recent commit to interpret walking into a shopkeeper as a "pay"
command was triggering in too many circumstances. Check to ensure
that the monster that we're walking into is a known shopkeeper
before activating the special case.

src/uhitm.c

@@ -479,7 +479,8 @@ do_attack(struct monst *mtmp)
                 char buf[BUFSZ];
 
                 if (!gc.context.travel && !gc.context.run)
-                    return ECMD_TIME | dopay();
+                    if (canspotmon(mtmp) && mtmp->isshk)
+                        return ECMD_TIME | dopay();
 
                 if (mtmp->mtame) /* see 'additional considerations' above */
                     monflee(mtmp, rnd(6), FALSE, FALSE);