CVE-2020-10730: lib ldb: Check if ldb_lock_backend_callback called twice

CVE-2020-10730: lib ldb: Check if ldb_lock_backend_callback called twice

Prevent use after free issues if ldb_lock_backend_callback is called
twice, usually due to ldb_module_done being called twice. This can happen if a
module ignores the return value from function a function that calls
ldb_module_done as part of it's error handling.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14364

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>