criu-util: do not double free and simplify xvstrcat

There is a bug, that if vsnprintf() wrote nothing to buffer:
that may be xstrcat(0, "%s", "") or something like that,
than vsnprintf's return value is 0, which will be lesser than
delta. The code before would do following:
o first cycle:
  1. relocate str to new (str is not allocated anymore)
  2. vsnprintf() retured 0, delta is greater.
o second cycle:
  1. relocate previously freed str to new..^C ^C
Segmentation fault (core dumped)

Weeell, I do think, we can do better job here.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>