test_objagg: Fix warning.

lib/test_objagg.c: In function ‘test_delta_action_item’:
./include/linux/printk.h:308:2: warning: ‘errmsg’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: David S. Miller <davem@davemloft.net>