ansicpg RTF command gives error

[fvanheeswijk]

We have an RTF with \ansicpg-535 as command, it gives the following error:

System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.ThrowHelper.ThrowKeyNotFoundException()
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at RtfPipe.TextEncoding.EncodingFromCodePage(Int32 codePage)
at RtfPipe.Parser.GetControlWord(String name, Int32 number)
at RtfPipe.Parser.d__18.MoveNext()
at RtfPipe.Parser.Parse()
at RtfPipe.Rtf.ToHtml(RtfSource source, XmlWriter writer, RtfHtmlSettings settings)
at RtfPipe.Rtf.ToHtml(RtfSource source, RtfHtmlSettings settings)

In joniles/rtfparserkit#13 I have found a similar issue, apparently negative numbers need to be treated differently.