mpegts: Do not try to write a PMT larger than SECTION_SIZE

Prevent out of array writes.

Similar to what Michael Niedermayer did to address the same issue.

Bug-Id: CVE-2014-2263
CC: libav-stable@libav.org

Signed-off-by: Diego Biurrun <diego@biurrun.de>
(cherry picked from commit e8049af1)

Conflicts:
	libavformat/mpegtsenc.c