codec: mpeg_audio: fix pts update
codec: mpeg_audio: fix pts update

Fixes #12722

(cherry picked from commit 72b03859fd8b715d9baeb5b378170d81517b01e8)

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2