Nil SOA causes panic if we compare it to incoming SOA (#291)

middleware/file/secondary.go

@@ -88,6 +88,9 @@ Transfer:
 	if serial == -1 {
 		return false, Err
 	}
+	if z.Apex.SOA == nil {
+		return true, Err
+	}
 	return less(z.Apex.SOA.Serial, uint32(serial)), Err
 }
 

middleware/file/secondary_test.go

@@ -84,9 +84,17 @@ func TestShouldTransfer(t *testing.T) {
 	z.origin = testZone
 	z.TransferFrom = []string{addrstr}
 
+	// when we have a nil SOA (initial state)
+	should, err := z.shouldTransfer()
+	if err != nil {
+		t.Fatalf("unable to run shouldTransfer: %v", err)
+	}
+	if !should {
+		t.Fatalf("shouldTransfer should return true for serial: %d", soa.serial)
+	}
 	// Serial smaller
 	z.Apex.SOA = test.SOA(fmt.Sprintf("%s IN SOA bla. bla. %d 0 0 0 0 ", testZone, soa.serial-1))
-	should, err := z.shouldTransfer()
+	should, err = z.shouldTransfer()
 	if err != nil {
 		t.Fatalf("unable to run shouldTransfer: %v", err)
 	}