audit: fix oops removing watch if audit disabled

Removing a watched file will oops if audit is disabled (auditctl -e 0).

To reproduce:
- auditctl -e 1
- touch /tmp/foo
- auditctl -w /tmp/foo
- auditctl -e 0
- rm /tmp/foo (or mv)
Signed-off-by: Tony Jones <tonyj@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/auditfilter.c

@@ -947,7 +947,7 @@ static void audit_update_watch(struct audit_parent *parent,
 
 		/* If the update involves invalidating rules, do the inode-based
 		 * filtering now, so we don't omit records. */
-		if (invalidating &&
+		if (invalidating && current->audit_context &&
 		    audit_filter_inodes(current, current->audit_context) == AUDIT_RECORD_CONTEXT)
 			audit_set_auditable(current->audit_context);