[C#] Fix CSharp compiler error when using uint32 length argument on variable length data types

Author: warrenfalk

main/java/uk/co/real_logic/sbe/generation/csharp/CSharpGenerator.java

@@ -376,7 +376,7 @@ private CharSequence generateVarData(final List<Token> tokens)
  " const int sizeOfLengthField = %3$d;\n" +
  " int limit = Limit;\n" +
  " _buffer.CheckLimit(limit + sizeOfLengthField);\n" +
- " int dataLength = _buffer.%4$sGet%5$s(limit);\n" +
+ " int dataLength = (int)_buffer.%4$sGet%5$s(limit);\n" +
  " int bytesCopied = Math.Min(length, dataLength);\n" +
  " Limit = limit + sizeOfLengthField + dataLength;\n" +
  " _buffer.GetBytes(limit + sizeOfLengthField, dst, dstOffset, bytesCopied);\n\n" +