mpegts: Define the section length with a constant

The specification says the value is expressed in 10 bits including
the 4-byte CRC.

(cherry picked from commit 694b7cd873f8b06af109036eff1ccd741afdd28e)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Conflicts:
	libavformat/mpegtsenc.c
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
(cherry picked from commit addbaf134836aea4e14f73add8c6d753a1373257)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>