Fix calculating checksum for single-TSDU sourced by application-vector.
Fix calculating checksum for single-TSDU sourced by application-vector.

jdelgadoalfonso/openpgm,jdelgadoalfonso/openpgm,jdelgadoalfonso/openpgm,jdelgadoalfonso/openpgm,jdelgadoalfonso/openpgm,jdelgadoalfonso/openpgm