block/file-posix: Fix laio_init() error handling crash bug

raw_aio_attach_aio_context() passes uninitialized Error *local_err by
reference to laio_init() via aio_setup_linux_aio().  When laio_init()
fails, it passes it on to error_setg_errno(), tripping error_setv()'s
assertion unless @local_err is null by dumb luck.

Fix by initializing @local_err properly.

Fixes: ed6e2161
Cc: Nishanth Aravamudan <naravamudan@digitalocean.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191130194240.10517-4-armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>