0077-proc/readproc.c: Harden fill_cgroup_cvt().

0077-proc/readproc.c: Harden fill_cgroup_cvt().

Check the return value of snprintf(), otherwise dst may point
out-of-bounds when it reaches the end of the dst_buffer (the snprintf()
always returns 1 in that case, even if there is not enough space left),
and vMAX becomes negative and is passed to snprintf() as a size_t.

---------------------------- adapted for newlib branch
. adapted via 'patch (without rejections)

Signed-off-by: Jim Warner <james.warner@comcast.net>