BUGFIX: Invalid write when extracting block header

Fixed issue wherein a block header was extracted from the
EntireHeader as:

memcpy(&blockHeader,&this->EntireHeader[offSet],sizeof(VariableHeader));

This was leading to an invalid write in the memory of blockHeader.
The intent of this line was:

memcpy(&blockHeader,&this->EntireHeader[offSet],sizeof(RankHeader));