Broken "--" operator on mappings

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=7384

Reported by Martin Karlgren marty@roxen.com

Pike v8.0 release 34 running Hilfe v3.5 (Incremental Pike Frontend)
> mapping m = ([ 4711: 5 ]);
> --m[4711];
(1) Result: 4710
> m;
(2) Result: ([ /* 1 element */
              4711: 5
            ])

Pike v7.8 release 883 running Hilfe v3.5 (Incremental Pike Frontend)
> mapping m = ([ 4711: 5 ]);
> --m[4711];
(1) Result: 4
> m;
(2) Result: ([ /* 1 element */
              4711: 4
            ])