livepatch: return -ENOMEM on ptr_id() allocation failure

Fixes the following smatch warning:

  lib/livepatch/test_klp_shadow_vars.c:47 ptr_id() warn: returning -1 instead of -ENOMEM is sloppy

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Petr Mladek <pmladek@suse.com>