test_objagg: Uninitialized variable in error handling

We need to set the error message on this path otherwise some of the
callers, such as test_hints_case(), print from an uninitialized pointer.

We had a similar bug earlier and set "errmsg" to NULL in the caller,
test_delta_action_item().  That code is no longer required so I have
removed it.

Fixes: 9069a381 ("lib: objagg: implement optimization hints assembly and use hints for object creation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>