nbd: assert that Error** is not NULL in nbd_iter_channel_error

nbd: assert that Error** is not NULL in nbd_iter_channel_error

All callers of nbd_iter_channel_error() pass the address of a
local_err variable, and only call this function if an error has
already occurred, using this function to propagate that error.
This is already implied by its name (local_err instead of the classic
errp), but it is worth additionally stressing this by adding an
assertion to make it part of the function contract.

The local_err parameter is not here to return information about
nbd_iter_channel_error failure. Instead it's assumed to be filled when
passed to the function. This is already stressed by its name
(local_err, instead of classic errp). Stress it additionally by
assertion.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20191205174635.18758-22-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>