pyldb: avoid segfault when adding an element with no name

pyldb: avoid segfault when adding an element with no name

We don't want to see this:

python3 -c "import sys
sys.path.insert(0, 'bin/python')
import ldb
m = ldb.Message()
e = ldb.MessageElement('q')
try:
    m.add(e)
except ldb.LdbError:
    pass
print(m)
"
Segmentation fault (core dumped)

instead we want this:

Traceback (most recent call last):
File "<string>", line 7, in <module>
ValueError: The element has no name

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Noel Power <npower@samba.org>
Autobuild-User(master): Noel Power <npower@samba.org>
Autobuild-Date(master): Tue Apr 23 19:03:35 UTC 2019 on sn-devel-144