qt: don't dereference error if there is no error

[hsitter]

blind fix for https://bugs.kde.org/show_bug.cgi?id=500691

not sure it even builds :)

[ximion]

Good in theory, but I thought this API was changed with the 1.0 release to behave a bit more predictably, where a FALSE return actually means "The error is set". I'll have a look if this can be fixed in a better way without an API break (if it isn't fixed already), but if not, we'll probably have to use this patch...

[ximion]

Is there any way I can reproduce this issue? I haven't managed to do that yet and there is no possible way for the function to return an empty error while also returning FALSE.
I reviewed that code, and there is no code path to that that I found. The patch to the Qt bindings papers over a potential issue elsewhere, and I would much rather find the root cause of the problem.

Also, knowing the versions of appstream and libxmlb that the reporter used would be helpful.