dvdwizardvob.cpp: check return value

Fixes Coverity CID 709275: Unchecked return value (CHECKED_RETURN)
Calling function "QDomDocument::setContent(QIODevice *, QString *, int *, int *)" without checking return value (as is done elsewhere 6 out of 7 times).
No check of the return value of "doc.setContent(&file, NULL, NULL, NULL)".
134            doc.setContent(&file);