apparmor: aa_getprocattr free procattr leak on format failure

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: 76a1d263aba3 ("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>