Fix run leak.

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
f193fd80cf (Refactor runs_avail.).

This resolves #493.

test/unit/run_quantize.c

@@ -111,7 +111,7 @@ TEST_BEGIN(test_monotonic)
 
 	floor_prev = 0;
 	ceil_prev = 0;
-	for (i = 1; i <= large_maxclass >> LG_PAGE; i++) {
+	for (i = 1; i <= chunksize >> LG_PAGE; i++) {
 		size_t run_size, floor, ceil;
 
 		run_size = i << LG_PAGE;