mongo-sync: Stop crashing when result verification fails

In case getLastError() fails (the db is locked or otherwise
inaccessible), the error output parameter was not touched, yet,
_mongo_sync_cmd_verify_result() tried to free it, and that resulted in a
crash.

Initialize the parameter to NULL, so that this does not happen.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>