Found with 'cppcheck --enable=all --inconclusive --std=posix' while
working with util-linux, which has a copy of this file.
[misc-utils/sd-daemon.c:363]: (style) Checking if unsigned variable \
'length' is less than zero.
[misc-utils/sd-daemon.c:366]: (style) Checking if unsigned variable \
'length' is less than zero.
References: http://www.spinics.net/lists/util-linux-ng/msg07031.html
strtol() and friends may set EINVAL if no conversion was performed, but
they are not required to do so. In practice they don't. We need to check
for it.
https://bugzilla.redhat.com/show_bug.cgi?id=870577
There is no point in clearing the bits of a "struct stat" when the very
next statement just calls stat or fstat to fill in that same memory.
[zj: two more places]
