h26xparse: Fix wrong NALU minimum length check
Fixes a problem where an EOS/EOB NALs placed at the end of an AU is detected as an other AU and create a buffer that does not have valid pts.
Since h26xparse does not consider EOS/EOB NALUs as complete NALUs when finding next NALU, so the EOS / EOB NALU is handled as a new frame and it will has the invalid pts.