iscsi: Don't use error_is_set() to suppress additional errors

Using error_is_set(errp) that way can sweep programming errors under
the carpet when we get called incorrectly with an error set.

Commit 24d3bd6 added a broken error path to iscsi_do_inquiry(): it
first calls error_setg(), then jumps to the preexisting error label,
where error_setg() gets called again, triggering an assertion failure.

Commit cbee81f fixed this by guarding the second error_setg() with an
error_is_set().

Replace this fix by a simpler and safer one: jump right behind the
second error_setg().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
