[3.0.x] subsdelay: handle NULL segment text

In some rare cases psz_text can be NULL, which would end up being passed to strlen, causing a crash.

Fix CD 524d077a-d613-4f2f-85b1-2a2af0747888

(cherry picked from commit 2322d55d) Signed-off-by: Marvin Scholz epirat07@gmail.com