wtperf: __wt_conn_dhandle_discard_single assert

wtperf -h /tmp/WT_TEST

-O /b/bostic/wiredtiger/bench/wtperf/runners/update-checkpoint-btree.wtperf

sweep-server: ../src/conn/conn_dhandle.c, 647: !final || ret == 0

(gdb) where

WT-6  0x0000000000481df3 in __wt_conn_dhandle_discard_single (

    session=0x802537000, dhandle=0x803647400, final=1)

    at ../src/conn/conn_dhandle.c:647

WT-7  0x0000000000481e58 in __wt_conn_dhandle_discard (conn=0x80250ce00)

    at ../src/conn/conn_dhandle.c:677

WT-8  0x000000000041209a in __wt_connection_close (conn=0x80250ce00)

    at ../src/conn/conn_open.c:92

WT-9  0x000000000040d2c9 in __conn_close (wt_conn=0x80250ce00, config=0x0)

    at ../src/conn/conn_api.c:573

WT-10 0x0000000000409cce in start_run (cfg=0x7fffffffd730)

    at ../../../bench/wtperf/wtperf.c:1869

WT-11 0x000000000040923b in start_all_runs (cfg=0x7fffffffd730)

    at ../../../bench/wtperf/wtperf.c:1652

WT-12 0x000000000040a745 in main (argc=5, argv=0x7fffffffd950)

    at ../../../bench/wtperf/wtperf.c:2086

(gdb) p final

$6 = 1

(gdb) p ret

$7 = 16

(gdb) p *dhandle

$8 = {rwlock = 0x80341d0c0, l = {sle_next = 0x8025a3400}, session_ref = 0, 

  session_inuse = 0, timeofdeath = 1402609850, 

  name_hash = 9596272081698085885, name = 0x80341d0d0 "file:test.wt", 

  checkpoint = 0x0, cfg = 0x80341e060, dsrc = 0x0, handle = 0x803667140,

{ statistics }, flags = 32}