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 initialises the block before writing content into it.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>
Merge pull request #1 from rdobson/CA-121295

CA-121295: Fix initialisation of memory used for writing tar header files
Merge pull request #2 from xapi-project/master

Pull fix for CA-121295 into trunk-ring3.