apparmor: Fix aa_label_build() error handling for failed merges

apparmor: Fix aa_label_build() error handling for failed merges

BugLink: https://bugs.launchpad.net/bugs/1837813
[ Upstream commit d6d478aee003e19ef90321176552a8ad2929a47f ]

aa_label_merge() can return NULL for memory allocations failures
make sure to handle and set the correct error in this case.

Reported-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>