qapi: Make string input and opts visitor require non-null input

The string input visitor tries to cope with null input.  Null input
isn't used anywhere, and isn't covered by tests.  Unsurprisingly, it
doesn't fully work: start_list() crashes because it passes the input
via parse_str() to strtoll() unchecked.

Make string_input_visitor_new() assert its argument isn't null, and
drop the code trying to deal with null input.

The opts visitor crashes when you try to actually visit something with
null input.  Make opts_visitor_new() assert its argument isn't null,
mostly for clarity.

qobject_input_visitor_new() already asserts its argument isn't null.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1488544368-30622-17-git-send-email-armbru@redhat.com>
