thumbnail: Fix heap-use-after-free in AudioCreator::create

There is a heap-use-after-free issue in AudioCreator::create resulting from storing the pointer to a temporary QByteArray's data() in a pointer and accessing it after the byte-array has been freed (when the the temporary object was created on is over).

This fixes it by moving the QByteArray onto the stack, thus making it not temporary anymore, keeping it around until its data isn't needed anymore.

This fixes https://bugs.kde.org/show_bug.cgi?id=469458