UPSTREAM: audit: Fix check of return value of strnlen_user()

(cherry picked from commit 0b08c5e5)

strnlen_user() returns 0 when it hits fault, not -1. Fix the test in
audit_log_single_execve_arg(). Luckily this shouldn't ever happen unless
there's a kernel bug so it's mostly a cosmetic fix.

CC: Paul Moore <pmoore@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paul Moore <pmoore@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Change-Id: I0eed84bbadd8fb49f81f22922468dface90acaba