unknownproto: check result from protowire.ConsumeFieldValue and return an error (#7770)

* unknownproto: check result from protowire.ConsumeFieldValue and return error

Given that protowire.ConsumeFieldValue returns -1 when it encounters an
error, perform a check for n < 0 and return the respectively obtained
error with context about the details.

Fixes an issue identified from a go-fuzz session, thanks to Ethan
Buchman and the IBC auditors from Informal Systems et al.

Fixes #7739.

* Address AlexanderBez's suggestions

* Use require in tests

* Add issue #7846 to TODO

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

codec/unknownproto/unknown_fields.go

@@ -92,6 +92,11 @@ func RejectUnknownFields(bz []byte, msg proto.Message, allowUnknownNonCriticals
 		// Skip over the bytes that store fieldNumber and wireType bytes.
 		bz = bz[m:]
 		n := protowire.ConsumeFieldValue(tagNum, wireType, bz)
+		if n < 0 {
+			err = fmt.Errorf("could not consume field value for tagNum: %d, wireType: %q; %w",
+				tagNum, wireTypeToString(wireType), protowire.ParseError(n))
+			return hasUnknownNonCriticals, err
+		}
 		fieldBytes := bz[:n]
 		bz = bz[n:]