proc: add a static buffer to prevent segv (v3)

A few of our functions use buffer and string functions.
All these functions require that a string contains '\0' at the end.
Before this patch we didn't guarantee that.

I've seen segmentation fault in parse_pid_stat_small.

v2: simplify code
v3: remove assignment, because it's redundant and a size of object file
become bigger.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>