ui-stats: cast pointer before checking for zero

ui-stats: cast pointer before checking for zero

We abuse the "void *util" field as a counter and recently started to
cast it to a uintptr_t to avoid risking nasal demons by performing
arithmetic on a void pointer.

However, compilers are also known to do "interesting" things if they
know that a pointer is or isn't NULL.  Make this safer by checking if
the counter (after casting) is non-zero rather than checking if the
pointer is non-null.

Signed-off-by: John Keeping <john@keeping.me.uk>