ul: Fix buffer overflow

ul: Fix buffer overflow

The text-utility ul can run into a buffer overflow on very long lines.
See this proof of concept how to reproduce the issue:

$ dd if=/dev/zero bs=1M count=10 | tr '\000' '\041' > poc.txt
$ echo -ne '\xe\x5f\x8\x5f\x61\x2\xf\x5f\x8\x5f' | dd of=poc.txt conv=notrunc
$ ul -i poc.txt > /dev/null # output would take ages
Segmentation fault
$ _

The problem manifests by using alloca with "maxcol", which can be as
large as INT_MAX, based on the input line.

A very long line (> 8 MB) with modes must be supplied to ul, as seen in
my proof of concept byte sequence above.

It is rather easy to fix this issue: allocate space on the heap instead.
maxcol could overflow here, but in that case no system will have enough
space to handle the request, properly ending ul through an err() call.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>