[C#] Incorrect offset type in verifier

[LacoVadkerti]

Offset type is converted to int16, but is computed as int position - int offset:

flatbuffers/net/FlatBuffers/FlatBufferVerify.cs

Line 243 in 94d6b80

short vtable = Convert.ToInt16(pos - ReadSOffsetT(verifier_buffer, pos));

[jtdavis777]

Hey @LacoVadkerti - would you be up for putting up a PR to fix this? maybe a test to show the problem as well?

[LacoVadkerti]

Hi @jtdavis777, OK, just give me some time to check how to do it.