[rt #70388] debug mymalloc() no longer builds a string, just stores the pointer

[rt #70388] debug mymalloc() no longer builds a string, just stores the pointer

The debug malloc() code would use snprintf() (or even worse,
sprintf()) to build a string to track when the memory allocation was
performed for later tracking.

Now we just store the pointers, which should come from __FILE__ and
hence be static.