apparmor: aa_getprocattr free procattr leak on format failure

aa_getprocattr() allocates the output string before rendering the label
into it. If the second aa_label_snxprint() call fails, the function
returned without freeing that allocation.

Free and clear the output pointer on the uncommon formatting failure path
before dropping the namespace reference.

Fixes: 76a1d263ab ("apparmor: switch getprocattr to using label_print fns()")
Reviewed-by: Tyler Hicks <code@thicks.com>
Reviewed-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/procattr.c

@@ -54,6 +54,8 @@ int aa_getprocattr(struct aa_label *label, char **string, bool newline)
 				FLAG_SHOW_MODE | FLAG_VIEW_SUBNS |
 				FLAG_HIDDEN_UNCONFINED);
 	if (len < 0) {
+		kfree(*string);
+		*string = NULL;
 		aa_put_ns(current_ns);
 		return len;
 	}