Fix #162: sadc crashes on a mtab file with really long lines

Fix #162: sadc crashes on a mtab file with really long lines

A segmentation fault may happen with "sadc -S DISK..." or
"sadc -S XDISK..." when lines longer than 512 bytes are read from
/etc/mtab.

Such lines are possible for instance when overlay2 filesystem
with docker is used. In such a case a single mtab entry can look
like this (note that new line characters were added for readability,
the original entry contained only one '\n' at the end):

overlay /var/lib/docker/overlay2/f23d2377a67b9ab1b49555ecd09b2ccdc03
7e0ee5d9e54f87e59f07f4398e71f/merged overlay rw,relatime,lowerdir=/v
ar/lib/docker/overlay2/l/L6VKIYXWBQSJ5R7V35SS43R6Y6:/var/lib/docker/
overlay2/l/UCCHKGXUJPWCMLHR36IZJNNIQP:/var/lib/docker/overlay2/l/RKV
YEXD2FH65FTMK76RDWPLESX:/var/lib/docker/overlay2/l/DX4JZRKTFP2GOO4V6
OWQ6CPJFY:/var/lib/docker/overlay2/l/6CYNWDKADUPPDZJ5IHOH2R7Y5S:/var
/lib/docker/overlay2/l/JTPINUZIATXADL6XWFHG2OYGSF:/var/lib/docker/ov
erlay2/l/OTSTIV5TTRHF4IUD7BODQ2FUON:/var/lib/docker/overlay2/l/QFNH3
EFS5EZGRTC4DPHU3PJ4TU:/var/lib/docker/overlay2/l/ZOOUKT2E5U4CSLP57Z7
MXYX5CD:/var/lib/docker/overlay2/l/3LUU6IDR2HWPTVBARC5K6XSMRC:/var/l
ib/docker/overlay2/l/XOHYBP4RWXQKQZ43I7JKG24KE4:/var/lib/docker/over
lay2/l/MN5M5B7AY5LPXQQC6V2MBJWWBF:/var/lib/docker/overlay2/l/3DRMKQ3
4AIZD2AROU3TVK3OCUT:/var/lib/docker/overlay2/l/73ZXDHBV6C53Q3SPXA57E
OLGHU:/var/lib/docker/overlay2/l/C2IZBQ55EUTGEAAORSLE73ZPNM:/var/lib
/docker/overlay2/l/ITHARNV7RPWN5S3BCZ2QDMZIMJ:/var/lib/docker/overla
y2/l/TQKUV4LEG4AFUUCMFHHRLDBHAH:/var/lib/docker/overlay2/l/N75JZWPPD
EKJ4DTN4GMEGTDIZL:/var/lib/docker/overlay2/l/QGUUYAETPMK643DG3AKWJAI
IZA,upperdir=/var/lib/docker/overlay2/f23d2377a67b9ab1b49555ecd09b2c
cdc037e0ee5d9e54f87e59f07f4398e71f/diff,workdir=/var/lib/docker/over
lay2/f23d2377a67b9ab1b49555ecd09b2ccdc037e0ee5d9e54f87e59f07f4398e71
f/work 0 0

The crash occurs in the get_filesystem_nr() and read_filesystem()
functions which call strchr(line, ' ') but fail to check if the result
is not NULL.

This patch adds this check, and when a single mtab entry requires more
that one call to fgets() (i.e. the entry is longer than 512 bytes), it
ignores outcome of the second and following calls.

Bugs-Debian: https://bugs.debian.org/872926
Signed-off-by: Robert Luberda <robert@debian.org>
Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>