Smack: handle zero-length security labels without panic

Smack: handle zero-length security labels without panic

Zero-length security labels are invalid but kernel should handle them.

This patch fixes kernel panic after setting zero-length security labels:

And after writing zero-length string into smackfs files syslog and onlycp:

The problem is caused by brain-damaged logic in function smk_parse_smack()
which takes pointer to buffer and its length but if length below or equal zero
it thinks that the buffer is zero-terminated. Unfortunately callers of this
function are widely used and proper fix requires serious refactoring.

Change-Id: Icf462c7ac2464d86093fa42a0ed012281b952571
Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>