SystemTools: Revert "Reduce scope of 'buf' variable in CollapseFullPath"

Revert commit 6e36d909 (SystemTools: Reduce scope of 'buf' variable in
CollapseFullPath, 2019-06-18).  A pointer to the buffer is returned by
`Getcwd` and stored in `res_in_base`.  The buffer must remain valid for
the lifetime of that variable.