mpeg12: do not decode extradata more than once.

Fixes CVE-2012-2803.

(cherry picked from commit 582368626188c070d4300913c6da5efa4c24cfb2)
(cherry picked from commit 301761792a693a1f3303a2af34a0fb066a03c10c)

Conflicts:

	libavcodec/mpeg12.c