(ITS#8537) liblmdb errors with data=NULL and non-matching SET_KEY

[h.b.furuseth@usit.uio.no]

Full_Name: Hallvard B Furuseth
Version: mdb.master, fa83b25ec536ac2642f8e0d3c6bf587008513ef4
OS: Linux x86_64
URL: ftp://ftp.openldap.org/incoming/Hallvard-Furuseth-161201.c
Submission from: (NULL) (129.240.203.186)
Submitted by: hallvard


liblmdb can crash or give an EINVAL in some cases after it
forgets to initialize an xcursor on a sub-page:

To fail the init, use next/prev with data=NULL, or SET_KEY
to a non-existe k key (so it gets positioned on next key).
Then try to use the cursor in another operation.

Test program enclosed.