flacparse: Avoid integer overflow in available data check for image tags

flacparse: Avoid integer overflow in available data check for image tags

If the image length as stored in the file is some bogus integer then
adding it to the current byte readers position can overflow and wrongly
have the check for enough available data succeed.

This then later can cause NULL pointer dereferences or out of bounds
reads/writes when actually reading the image data.

Fixes ZDI-CAN-20775
Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2661

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4897>