mpjpeg: probe should require same constraints as packet reader - both proper content-type and content-size must be present

return AVPROBE_SCORE_MAX, rather than random positive number on success
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

avformat/mpjpegdec: Allow mpjpeg headers parsing to succeed upon EOF, if required headers are present
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

Bug-Id: CID 1308152

The first check is done without the AVIOContext, so alloc it only if said check succeeds
Signed-off-by: NJames Almer <jamrial@gmail.com>
Signed-off-by: NJanne Grunau <janne-libav@jannau.net>

The first check is done without the AVIOContext, so alloc it only if said check succeeds
Reviewed-by: NMichael Niedermayer <michaelni@gmx.at>
Signed-off-by: NJames Almer <jamrial@gmail.com>

Signed-off-by: NJames Almer <jamrial@gmail.com>

Support only streams with Content-Length.
Signed-off-by: NLuca Barbato <lu_zero@gentoo.org>