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

When refactoring to allocate memory using the cstruct libray, an explict scrub of the allocation was not made.

This fix uses the 'zero_block' function to allocate a 512 block, into which the header is written.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>

I've tested the patch with XS and this fixes CA-121295.