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 <anthony@codemonkey.ws>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>

qom/object.c

@@ -1612,12 +1612,16 @@ static void property_get_str(Object *obj, Visitor *v, void *opaque,
 {
     StringProperty *prop = opaque;
     char *value;
+    Error *err = NULL;
 
-    value = prop->get(obj, errp);
-    if (value) {
-        visit_type_str(v, &value, name, errp);
-        g_free(value);
+    value = prop->get(obj, &err);
+    if (err) {
+        error_propagate(errp, err);
+        return;
     }
+
+    visit_type_str(v, &value, name, errp);
+    g_free(value);
 }
 
 static void property_set_str(Object *obj, Visitor *v, void *opaque,