avformat/apngdec: set pts to AV_NOPTS_VALUE

And let the generic code figure out the proper PTS. This is needed because apng
does not provide seek functions, but after a generic seek (e.g. to file start)
timestamps are not reset which causes broken timestamps when looping apngs,
like in ticket #6121.

Signed-off-by: Marton Balint <cus@passwd.hu>