qapi: Fix crash when 'any' or 'null' parameter is missing

Unlike the other visit methods, visit_type_any() and visit_type_null()
neglect to check whether qmp_input_get_object() succeeded.  They crash
when it fails.  Reproducer:

{ "execute": "qom-set",
  "arguments": { "path": "/machine", "property": "rtc-time" } }

Will crash with:

qapi/qapi-visit-core.c:277: visit_type_any: Assertion `!err != !*obj'
failed

Broken in commit 5c678ee.  Fix by adding the missing error checks.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20160922203927.28241-3-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message rephrased]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
