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>

ui-stats.c

@@ -184,9 +184,9 @@ static void add_commit(struct string_list *authors, struct commit *commit,
 	period->trunc(date);
 	tmp = xstrdup(period->pretty(date));
 	item = string_list_insert(items, tmp);
-	if (item->util)
-		free(tmp);
 	counter = (uintptr_t *)&item->util;
+	if (*counter)
+		free(tmp);
 	(*counter)++;
 
 	authorstat->total++;