The value from tar header is unsigned int; keep it casted to
unsigned int and size_t instead of (signed) int, otherwise
the comparisons fail to do their job properly. Additionally check
entry.size against SSIZE_MAX so the rounding up later on is
guaranteed to not overflow.

Fixes CVE-2017-9669 and CVE-2017-9671.
Reported-by: Ariel Zelivansky from Twistlock