lavc/srtenc: fix invalid read in case of SubRip.

Regression since 6057de19b. The ptr-2 is used to eat the \r\n and add
the position information on the timing line. This can't be done in case
of SubRip where the timing isn't present in the payload.

Note that we can't use yet the side data to transmit the position
information since the encode subtitles API is still using buffer+size
instead AVPacket as input.
