avcodec/exr: Check for duplicate channel index

Fixes: Out of memory
Fixes: 11582/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-5730204559867904

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/exr.c

@@ -1463,6 +1463,11 @@ static int decode_header(EXRContext *s, AVFrame *frame)
                     }
                     s->pixel_type                     = current_pixel_type;
                     s->channel_offsets[channel_index] = s->current_channel_offset;
+                } else if (channel_index >= 0) {
+                    av_log(s->avctx, AV_LOG_ERROR,
+                            "Multiple channels with index %d.\n", channel_index);
+                    ret = AVERROR_INVALIDDATA;
+                    goto fail;
                 }
 
                 s->channels = av_realloc(s->channels,