avformat/img2dec: allocate and clear padding area for probe correctly

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/img2dec: avoid seekback on probeing

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/img2dec: set AVProbeData size correctly on corner cases of tiny files

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>