3.9.1 FieldOptions cause an IndexOutOfRangeException when accessing the Descriptor in c# (works in 3.8.0)

[Pete Townsend]

If I use Version 3.9.1 of the Google.Protobuf and access the Descriptor for my message I get an IndexOutOfRangeException - however if I switch back to 3.8.0 it;s fine.

Example:

### File 1 "test.proto"

syntax = "proto3";

import "google/protobuf/wrappers.proto";

import "protos/myoption.proto";

message Test {

  string test_id = 1 [(myoption) = "test"];

}

### End File 1

### File 2 "myoption.proto"

syntax = "proto3";

import "google/protobuf/descriptor.proto";

extend google.protobuf.FieldOptions {

string myoption = 50782;

}

### End File 2