tests/heap_cmd: fix -Wuse-after-free
tests/heap_cmd: fix -Wuse-after-free

Strictly speaking, this is not actually a use after free, as only the
address of the freed memory chunk is printed. The freed memory is not
accesses. However, this is more idiomatic this way.

jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT