protoprint: when sorting elements, first value must be zero in proto3 file(#582)

Author: japersik

desc/protoprint/print.go

@@ -2137,11 +2137,20 @@ func (a elementAddrs) Less(i, j int) bool {
 		return vi.GetNumber() < vj.GetNumber()
 
 	case *desc.EnumValueDescriptor:
-		// enum values ordered by number then name
+		// enum values ordered by number then name,
+		// but first value number must be 0 in proto3
 		vj := dj.(*desc.EnumValueDescriptor)
 		if vi.GetNumber() == vj.GetNumber() {
 			return vi.GetName() < vj.GetName()
 		}
+		if vi.GetFile().IsProto3() {
+			if vj.GetNumber() == 0 {
+				return false
+			}
+			if vi.GetNumber() == 0 {
+				return true
+			}
+		}
 		return vi.GetNumber() < vj.GetNumber()
 
 	case *descriptorpb.DescriptorProto_ExtensionRange: