CA-121295: Fix initialisation of memory used for writing tar header files

When refactoring to allocate memory using the cstruct libray, an explicit scrub of the allocation was not made.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>

I've tested the patch with XS and this fixes CA-121295.

- Fixed to address Dave Scott's comment about the use of zero_block - which is not returning a new block on each call.