apparmor: Fix uninitialized value in aa_split_fqname

Syzkaller reported a OOB-read with the stacktrace below. This occurs
inside __aa_lookupn_ns as `n` is not initialized. `n` is obtained from
aa_splitn_fqname. In cases where `name` is invalid, aa_splitn_fqname
returns without initializing `ns_name` and `ns_len`.

Fix this by always initializing `ns_name` and `ns_len`.

	__dump_stack lib/dump_stack.c:77 [inline]
	dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
	print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
	kasan_report_error mm/kasan/report.c:354 [inline]
	kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
	__asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
	memcmp+0xe3/0x160 lib/string.c:861
	strnstr+0x4b/0x70 lib/string.c:934
	__aa_lookupn_ns+0xc1/0x570 security/apparmor/policy_ns.c:209
	aa_lookupn_ns+0x88/0x1e0 security/apparmor/policy_ns.c:240
	aa_fqlookupn_profile+0x1b9/0x1010 security/apparmor/policy.c:468
	fqlookupn_profile+0x80/0xc0 security/apparmor/label.c:1844
	aa_label_strn_parse+0xa3a/0x1230 security/apparmor/label.c:1908
	aa_label_parse+0x42/0x50 security/apparmor/label.c:1943
	aa_change_profile+0x513/0x3510 security/apparmor/domain.c:1362
	apparmor_setprocattr+0xaa4/0x1150 security/apparmor/lsm.c:658
	security_setprocattr+0x66/0xc0 security/security.c:1298
	proc_pid_attr_write+0x301/0x540 fs/proc/base.c:2555
	__vfs_write+0x119/0x9f0 fs/read_write.c:485
	vfs_write+0x1fc/0x560 fs/read_write.c:549
	ksys_write+0x101/0x260 fs/read_write.c:598
	__do_sys_write fs/read_write.c:610 [inline]
	__se_sys_write fs/read_write.c:607 [inline]
	__x64_sys_write+0x73/0xb0 fs/read_write.c:607
	do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
	entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 3b0aaf58

 ("apparmor: add lib fn to find the "split" for fqnames")
Reported-by:  <syzbot+61e4b490d9d2da591b50@syzkaller.appspotmail.com>
Signed-off-by: Zubin Mithra <zsm@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: John Johansen <john.johansen@canonical.com>