qapi-commands.py: Don't call the output marshal on error

Today we generate something like this:

    int qmp_marshal_input_query_foo(...)

        ...

        retval = qmp_query_foo(errp);
        qmp_marshal_output_query_foo(retval, ret, errp);

        ...

However, if qmp_query_foo() fails 'retval' will probably be NULL,
which can cause a segfault as not all visitors check if 'retval'
is valid.

This commit fixes that by changing the code generator to only
call the output marshal if qmp_query_foo() succeeds, like this:

    retval = qmp_query_foo(errp);
    if (!error_is_set(errp)) {
        qmp_marshal_output_query_foo(retval, ret, errp);
    }

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
