Fix arena_run_first_best_fit() to search all potentially non-empty
runs_avail heaps, rather than ignoring the heap that contains runs
larger than large_maxclass, but less than chunksize.
This fixes a regression caused by
f193fd80cf1f99bce2bc9f5f4a8b149219965da2 (Refactor runs_avail.).
This resolves#493.