Fix integer overflows.

Fix integer overflows.

The data structures linkhash and printbuf are limited to 2 GB in size
due to a signed integer being used to track their current size.

If too much data is added, then size variable can overflow, which is
an undefined behaviour in C programming language.

Assuming that a signed int overflow just leads to a negative value,
like it happens on many sytems (Linux i686/amd64 with gcc), then
printbuf is vulnerable to an out of boundary write on 64 bit systems.