Revert "mpegtspacketizer: memcmp potentially seen_before data"

This reverts commit fcad4cc6.

This was wrong is so many ways.

• The memcmp was badly used (it should use == 0 to check the data is identical, and not != 0)
• There was no boundary checks on the present stream section_data when passing it to memcmp.
• The return value should have been TRUE (i.e. we have done all checks, none of them failed, therefore the section has been seen before)
• stream->section_data would always be NULL if the section had already been processed

Fixes #1559 (closed) and #1558 (closed)

Also mpegts: Check is program is identical before updating