lnwire: fix decoding for initial zero sid

This fixes a decoding error when the list of short channel ids within a
QueryShortChanIDs message started with a zero sid.

BOLT-0007 specifies that lists of short channel ids should be sorted in
ascending order. Previously, this was checked within lnwire by comparing
two consecutive sids in the list, starting at the empty (zero) sid.

This meant that a list that started with a zero sid couldn't be decoded
since the first element would _not_ be greater than the last one
(namely: also zero).

Given that one can only check for ordering starting at the second
element, we add a check to ensure the proper behavior.

A unit test is also added to ensure no future regressions on this
behavior.

lnwire/query_short_chan_ids_test.go

@@ -49,6 +49,7 @@ var (
 // TestQueryShortChanIDsUnsorted tests that decoding a QueryShortChanID request
 // that contains duplicate or unsorted ids returns an ErrUnsortedSIDs failure.
 func TestQueryShortChanIDsUnsorted(t *testing.T) {
+
 	for _, test := range unsortedSidTests {
 		test := test
 		t.Run(test.name, func(t *testing.T) {
@@ -73,3 +74,48 @@ func TestQueryShortChanIDsUnsorted(t *testing.T) {
 		})
 	}
 }
+
+// TestQueryShortChanIDsZero ensures that decoding of a list of short chan ids
+// still works as expected when the first element of the list is zero.
+func TestQueryShortChanIDsZero(t *testing.T) {
+	testCases := []struct {
+		name     string
+		encoding ShortChanIDEncoding
+	}{
+		{
+			name:     "plain",
+			encoding: EncodingSortedPlain,
+		}, {
+			name:     "zlib",
+			encoding: EncodingSortedZlib,
+		},
+	}
+
+	testSids := []ShortChannelID{
+		NewShortChanIDFromInt(0),
+		NewShortChanIDFromInt(10),
+	}
+
+	for _, test := range testCases {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			req := &QueryShortChanIDs{
+				EncodingType: test.encoding,
+				ShortChanIDs: testSids,
+				noSort:       true,
+			}
+
+			var b bytes.Buffer
+			err := req.Encode(&b, 0)
+			if err != nil {
+				t.Fatalf("unable to encode req: %v", err)
+			}
+
+			var req2 QueryShortChanIDs
+			err = req2.Decode(bytes.NewReader(b.Bytes()), 0)
+			if err != nil {
+				t.Fatalf("unexpected decoding error: %v", err)
+			}
+		})
+	}
+}