UBUNTU: SAUCE: LSM stacking: check for invalid zero sized writes

UBUNTU: SAUCE: LSM stacking: check for invalid zero sized writes

BugLink: http://bugs.launchpad.net/bugs/1720779
BugLink: http://bugs.launchpad.net/bugs/1763062
Writing zero bytes to /proc/$pid/task/$pid/attr/context via
security_setprocattr cause an oops in memcpy_erms. Fix this by
checking for zero size and returning -EINVAL for this invalid
write size.

Detected by running stress-ng --procfs 0

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>