(ITS#8310) LMDB cursor_del followed by MDB_NEXT can trigger assert crash

[steven.lang@hgst.com]

Full_Name: Steven Lang
Version: LMDB 0.9.16
OS: Ubuntu 14.04
URL: ftp://ftp.openldap.org/incoming/steven-lang-151112.tar.gz
Submission from: (NULL) (50.76.49.9)


This seems to be a variation on ITS#8258 - the base cause is the same; an
mdb_cursor_del causes the tree to grow in depth, the cursor is not updated with
the new depth, and the next cursor op on the cursor asserts.

I have a DB and short C file which triggers the assert in a simple manner.