avformat/mpjpegdec: fix strict boundary search string

avformat/mpjpegdec: fix strict boundary search string

According to RFC1341, the multipart boundary indicated by the
Content-Type header must be prepended by CRLF + "--", and followed
by CRLF. In the case of strict MIME header boundary handling, the
"--" was forgotten to add.

Fixes trac #7921.

A side effect is that this coincidentally breaks enforcement of
strict MIME headers against servers running motion < 3.4.1, where
the boundary announcement in the HTTP headers incorrectly used the
prefix "--", which exactly matched this bug's behavior.

Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>