exr: fix header parsing

the header in the sample provided for ticket #1306 is not parsed correctly and thus
ffmpeg tries to decode the sample instead of abording the decoding.

I tested it with two other exr samples I have - one float, one half float - and
they still decode correctly.

Signed-off-by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>