Fix psset_pick_purge when last candidate with index 0 dirtiness is ineligible

psset_pick_purge used max_bit-- after rejecting a time-ineligible
candidate, which caused unnecessary re-scanning of the same bitmap
and makes assert fail in debug mode) and a size_t underflow
when the lowest-index entry was rejected.  Use max_bit = ind - 1
to skip directly past the rejected index.

src/psset.c

@@ -417,7 +417,10 @@ psset_pick_purge(psset_t *psset, const nstime_t *now) {
 		if (nstime_compare(tm_allowed, now) <= 0) {
 			return ps;
 		}
-		max_bit--;
+		if (ind == 0) {
+			break;
+		}
+		max_bit = ind - 1;
 	}
 	/* No page is ready yet */
 	return NULL;