middleware/file: consider no SOA a fatal error (#734)

Don't load a zone with a SOA record, barf with 'no SOA record' error.

middleware/file/file.go

@@ -126,13 +126,17 @@ func Parse(f io.Reader, origin, fileName string, serial int64) (*Zone, error) {
 				if s.Serial == uint32(serial) { // same zone
 					return nil, fmt.Errorf("no change in serial: %d", serial)
 				}
+				seenSOA = true
 			}
-			seenSOA = true
 		}
 
 		if err := z.Insert(x.RR); err != nil {
 			return nil, err
 		}
 	}
+	if !seenSOA {
+		return nil, fmt.Errorf("file %q has no SOA record", fileName)
+	}
+
 	return z, nil
 }