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.

tests/heap_cmd/main.c

@@ -43,8 +43,8 @@ static int free_cmd(int argc, char **argv)
 
     unsigned int p = strtoul(argv[1], NULL, 16);
     void *ptr = (void *)p;
+    printf("freeing %p\n", ptr);
     free(ptr);
-    printf("freed %p\n", ptr);
     return 0;
 }