lib-ldb_ldif: Stop processing if ldb_message element name is NULL

I have hit this while using Python bindings for testing and
forgot to pass 'name' argument to MessageElement constructor
Passing NULL as 'name' is obviously a mistake and this commits
aims to warn about such unintetional mistakes.

Signed-off-by: Kamen Mazdrashki <kamenim@samba.org>
lib-pyldb: Avoid crash when copying MessageElements between Python Message objects

This patch allows for following snipets in Python:
  res = ldb.search(...)
  m_from = res[0]
  m_to = Message()
  m_to.add(m_from["attrName"])

The problem previously is that we are trying to reference a
ldb_message_element that may not be a memory context on its own.
For instance, when search request from above example returns
Messages with more than one attribute, this leads immediately
to "Bad talloc magic value" crash, every message element beside
the first one is not a memory context

Signed-off-by: Kamen Mazdrashki <kamenim@samba.org>