Fix root zone handling.
    
    syncrepl_update() was buggy in a way which could cause accidental zone removal.
    
    Test case: A server with two zones: '.' and 'test.'
    
    Zone '.':
    .     NS ns1.test.
    .     NS ns2.test.
    test. NS ns1.test.
    test. NS ns2.test.
    
    Zone 'test.':
    test.     NS ns1.test.
    test.     NS ns2.test.
    ns1.test. A  192.0.2.1
    ns2.test. A  192.0.2.2
    
    Removing whole name 'test.' from zone '.' will cause removal of zone 'test.'
    instead of removing NS records from zone '.'.
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>