Port upstream LibArchive commit "compute string pointers after
concatenation" (2014-09-25) and commit "Move variables to top of
function for non-C99 compilers" (2014-11-15) to our CMake copy.
Otherwise we may compute a pointer to memory that is about to be freed
and then compute a bad size to give to CryptGenRandom.
Inspired-by: Tim Kientzle <kientzle@gmail.com>
1f33b45d5d