Add unit tests for ExtractAddressFromReverse (#328)

This fix adds unit tests for `ExtractAddressFromReverse`. It also
updates the implementation of `ExtractAddressFromReverse` so that
only a valid IPv4 or IPv6 address (in string format) is returned.

If the addresses is not valid and can not be handled by golang's
`net.ParseIP` then an empty string (`""`) will be returned.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/pkg/dnsutil/reverse_test.go

@@ -0,0 +1,51 @@
+package dnsutil
+
+import (
+	"testing"
+)
+
+func TestExtractAddressFromReverse(t *testing.T) {
+	tests := []struct {
+		reverseName     string
+		expectedAddress string
+	}{
+		{
+			"54.119.58.176.in-addr.arpa.",
+			"176.58.119.54",
+		},
+		{
+			".58.176.in-addr.arpa.",
+			"",
+		},
+		{
+			"b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.in-addr.arpa.",
+			"",
+		},
+		{
+			"b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.",
+			"2001:db8::567:89ab",
+		},
+		{
+			"d.0.1.0.0.2.ip6.arpa.",
+			"",
+		},
+		{
+			"54.119.58.176.ip6.arpa.",
+			"",
+		},
+		{
+			"NONAME",
+			"",
+		},
+		{
+			"",
+			"",
+		},
+	}
+	for i, test := range tests {
+		got := ExtractAddressFromReverse(test.reverseName)
+		if got != test.expectedAddress {
+			t.Errorf("Test %d, expected '%s', got '%s'", i, test.expectedAddress, got)
+		}
+	}
+}