core dump during checkpoint

#0  0x00000000004de394 in __sync_file (session=0x80351ea00, syncop=1)

    at src/btree/bt_sync.c:155

#1  0x00000000004dde81 in __wt_cache_op (session=0x80351ea00, 

    ckptbase=0x80cd49c00, op=1) at src/btree/bt_sync.c:277

#2  0x00000000004a1de5 in __checkpoint_worker (session=0x80351ea00, 

    cfg=0x7ffffddeedc0, is_checkpoint=true, need_tracking=true)

    at src/txn/txn_ckpt.c:1100

#3  0x00000000004a0f3e in __wt_checkpoint (session=0x80351ea00, 

    cfg=0x7ffffddeedc0) at src/txn/txn_ckpt.c:1191

#4  0x00000000004a35cc in __checkpoint_apply (session=0x80351ea00, 

    cfg=0x7ffffddeedc0, op=0x4a0e80 <__wt_checkpoint>)

    at src/txn/txn_ckpt.c:184

#5  0x00000000004a065d in __txn_checkpoint (session=0x80351ea00, 

    cfg=0x7ffffddeedc0) at src/txn/txn_ckpt.c:507

#6  0x000000000049f893 in __wt_txn_checkpoint (session=0x80351ea00, 

    cfg=0x7ffffddeedc0) at src/txn/txn_ckpt.c:668

#7  0x000000000048b921 in __session_checkpoint (wt_session=0x80351ea00, 

    config=0x0) at src/session/session_api.c:1067

#8  0x0000000000408334 in ops (arg=0x80347cdc0) at ops.c:367

(gdb) l

151				 * Mark the tree dirty: the checkpoint marked it clean

152				 * and we can't skip future checkpoints until this page

153				 * is written.

154				 */

155				if (!WT_PAGE_IS_INTERNAL(page) &&

156				    F_ISSET(txn, WT_TXN_HAS_SNAPSHOT) &&

157				    WT_TXNID_LT(txn->snap_max, mod->first_dirty_txn)) {

158					__wt_page_modify_set(session, page);

159					continue;

(gdb) p page->type

$6 = 4 '\004'

(gdb) p/x txn->flags

$7 = 0x4c

(gdb) p mod

$8 = (WT_PAGE_MODIFY *) 0x0

(gdb) p page->modify

$9 = (WT_PAGE_MODIFY *) 0x8101c3740

(gdb) p txn->snap_max

$10 = 880303

(gdb) p page->modify->first_dirty_txn

$11 = 882711

(gdb) l 131,137

131				page = walk->page;

132				mod = page->modify;

133	

134				/* Skip clean pages. */

135				if (!__wt_page_is_modified(page))

136					continue;