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>

libavcodec/exr.c

@@ -198,7 +198,7 @@ static int decode_frame(AVCodecContext *avctx,
             if (!variable_buffer_data_size)
                 return -1;
 
-            channel_list_end = buf + variable_buffer_data_size + 4;
+            channel_list_end = buf + variable_buffer_data_size;
             while (channel_list_end - buf >= 19) {
                 int current_bits_per_color_id = -1;
                 int channel_index = -1;