qom: Fix invalid error check in property_get_str()

qom: Fix invalid error check in property_get_str()

When a function returns a null pointer on error and only on error, you
can do

    if (!foo(foos, errp)) {
        ... handle error ...
    }

instead of the more cumbersome

    Error *err = NULL;

    if (!foo(foos, &err)) {
        error_propagate(errp, err);
        ... handle error ...
    }

A StringProperty's getter, however, may return null on success!  We
then fail to call visit_type_str().

Screwed up in 6a146eb, v1.1.

Fails tests/qom-test in my current, heavily hacked QAPI branch.  No
reproducer for master known (but I didn't look hard).

Cc: Anthony Liguori <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Cc: [email protected]
Signed-off-by: Andreas Färber <[email protected]>