Mp4: validate sync sample values in stss atom.

Mp4: validate sync sample values in stss atom.

Per ISO 14496-12 Section 8.6.2, sync sample numbers must be 1-based.
A zero-valued stss entry caused ngx_http_mp4_seek_key_frame() to
return a key_prefix exceeding the samples consumed in the forward
stts pass, which led the backward loop in ngx_http_mp4_crop_stts_data()
to walk past the beginning of the stts data buffer.

The fix validates each stss entry in ngx_http_mp4_seek_key_frame()
and returns an error if a zero sync sample is encountered.  The
function signature is changed to return ngx_int_t so it can signal
errors to the caller.