Bug#800468: crashes in liblmdb on start-up

[Mario Blättermann <mario.blaettermann@gmail.com>]

Same behavior on a freshly installed Stretch. The gdb output:

(gdb) run
Starting program: /usr/bin/dolphin
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffe235a700 (LWP 14801)]
[New Thread 0x7fffdafbd700 (LWP 14802)]

Program received signal SIGSEGV, Segmentation fault.
0x00007fffead34960 in mdb_txn_begin ()
   from /usr/lib/x86_64-linux-gnu/liblmdb.so.0

It happened after I tried to config Dolphin to let the preview sidebar 
appear.

Best Regards,
Mario