Don't access unmapped pages when evicting a tree

Thread 1 "python" received signal SIGSEGV, Segmentation fault.

0x00007ffff5da7e63 in __wt_page_out (session=0x555555f18570, pagep=0x555556033990)

    at ../src/btree/bt_discard.c:100

100			(void)S2BT(session)->bm->map_discard(

(gdb) bt

#0  0x00007ffff5da7e63 in __wt_page_out (session=0x555555f18570, pagep=0x555556033990)

    at ../src/btree/bt_discard.c:100

#1  0x00007ffff5da7c78 in __wt_ref_out (session=0x555555f18570, ref=0x555556033990)

    at ../src/btree/bt_discard.c:44

#2  0x00007ffff5e753e7 in __wt_evict_file (session=0x555555f18570, 

    syncop=WT_SYNC_DISCARD) at ../src/evict/evict_file.c:128

#3  0x00007ffff5dda142 in __wt_cache_op (session=0x555555f18570, op=WT_SYNC_DISCARD)

    at ../src/btree/bt_sync.c:439

#4  0x00007ffff5e17b29 in __wt_conn_dhandle_close (session=0x555555f18570, 

    final=true, mark_dead=false) at ../src/conn/conn_dhandle.c:374

#5  0x00007ffff5e18fc5 in __wt_conn_dhandle_discard_single (session=0x555555f18570, 

    final=true, mark_dead=false) at ../src/conn/conn_dhandle.c:730

#6  0x00007ffff5e196af in __wt_conn_dhandle_discard (session=0x555555f18570)

    at ../src/conn/conn_dhandle.c:798

#7  0x00007ffff5e1f211 in __wt_connection_close (conn=0x555555f00ce0)

    at ../src/conn/conn_open.c:112

#8  0x00007ffff5e07e32 in __conn_close (wt_conn=0x555555f00ce0, config=0x0)

    at ../src/conn/conn_api.c:1147

#9  0x00007ffff61bff31 in _wrap_Connection_close (self=<optimised out>, 

    args=<optimised out>) at wiredtiger_wrap.c:8645