[3.0] codec: libass: fix inverted patch ratio

assert() may be disabled and you don't want to abort a whole process in case of a parsing issue. So check the offset from samples_avail(), that will return 0 (EOF) in case of a out of bounds read.

Patch already upstream.

(cherry picked from commit 8205482c) Signed-off-by: Thomas Guillem thomas@gllm.fr