mpegts: Define the section length with a constant

The specification says the value is expressed in 10 bits including
the 4-byte CRC.
mpegts: Do not try to write a PMT larger than SECTION_SIZE

Prevent out of array write.

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

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