fix undefined path_max for st_size zero

fix undefined path_max for st_size zero

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1792647
Bug-Debian: https://bugs.debian.org/909011
Reviewed-by: dod
The downstream 'path_max' patch in Debian sets the buffer size
for readlink() to the 'st_size' value obtained with lstat().

However, it might be zero for some symlinks in /proc on Linux
(notably /proc/self) leading to readlink() failing with EINVAL.

    $ strace -e lstat stat /proc/self 2>&1 \
        | grep -e lstat -e File: -e Size:
    lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
      File: /proc/self -> 30875
      Size: 0             Blocks: 0          IO Block: 1024   symbolic link

This causes readlink (tool) to files like /dev/stdin to fail,
which may link to /proc/self/fd/0 on containers or elsewhere.

Test-case:

    ubuntu@cosmic:~/node$
    $ strace -E LD_LIBRARY_PATH=/usr/local/lib/ -f -e lstat,readlink \
      node test/parallel/test-fs-realpath-pipe.js

With path_max:

    [pid 17785] lstat("/dev", {st_mode=S_IFDIR|0755, st_size=480, ...}) = 0
    [pid 17786] lstat("/dev/stdin", {st_mode=S_IFLNK|0777, st_size=15, ...}) = 0
    [pid 17788] lstat("/dev/stdin", {st_mode=S_IFLNK|0777, st_size=15, ...}) = 0
    [pid 17788] readlink("/dev/stdin", "/proc/self/fd/0", 15) = 15

    [pid 17785] lstat("/proc", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
    [pid 17786] lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
    [pid 17788] lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
    [pid 17788] readlink("/proc/self", 0x7f2a6c000b40, 0) = -1 EINVAL (Invalid argument)

Without path_max:

    [pid 18114] lstat("/dev", {st_mode=S_IFDIR|0755, st_size=480, ...}) = 0
    [pid 18114] lstat("/dev/stdin", {st_mode=S_IFLNK|0777, st_size=15, ...}) = 0
    [pid 18114] readlink("/dev/stdin", "/proc/self/fd/0", 4096) = 15

    [pid 18114] lstat("/proc", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
    [pid 18114] lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
    [pid 18114] readlink("/proc/self", "18114", 4096) = 5

    [pid 18114] lstat("/proc/18114", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
    [pid 18114] lstat("/proc/18114/fd", {st_mode=S_IFDIR|0500, st_size=0, ...}) = 0
    [pid 18114] lstat("/proc/18114/fd/0", {st_mode=S_IFLNK|0700, st_size=64, ...}) = 0
    [pid 18114] readlink("/proc/18114/fd/0", "socket:[199607]", 4096) = 15

With this patch on top of path_max:

    [pid 18433] lstat("/dev", {st_mode=S_IFDIR|0755, st_size=480, ...}) = 0
    [pid 18433] lstat("/dev/stdin", {st_mode=S_IFLNK|0777, st_size=15, ...}) = 0
    [pid 18433] lstat("/dev/stdin", {st_mode=S_IFLNK|0777, st_size=15, ...}) = 0
    [pid 18433] readlink("/dev/stdin", "/proc/self/fd/0", 15) = 15

    [pid 18433] lstat("/proc", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
    [pid 18433] lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
    [pid 18433] lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
    [pid 18433] readlink("/proc/self", "18433", 256) = 5

    [pid 18433] lstat("/proc/18433", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
    [pid 18433] lstat("/proc/18433/fd", {st_mode=S_IFDIR|0500, st_size=0, ...}) = 0
    [pid 18433] lstat("/proc/18433/fd/0", {st_mode=S_IFLNK|0700, st_size=64, ...}) = 0
    [pid 18433] lstat("/proc/18433/fd/0", {st_mode=S_IFLNK|0700, st_size=64, ...}) = 0
    [pid 18433] readlink("/proc/18433/fd/0", "socket:[191351]", 64) = 15
Reviewed-by: dod
Gbp-Pq: Name path_max_zero_st_size