avcodec/exr: fix channel detection

libavcodec/exr.c

@@ -1382,15 +1382,17 @@ static int decode_header(EXRContext *s)
                     return AVERROR_PATCHWELCOME;
                 }
 
-                if (channel_index >= 0) {
-                    if (s->pixel_type != EXR_UNKNOWN &&
-                        s->pixel_type != current_pixel_type) {
-                        av_log(s->avctx, AV_LOG_ERROR,
-                               "RGB channels not of the same depth.\n");
-                        return AVERROR_INVALIDDATA;
+                if (s->channel_offsets[channel_index] == -1){/* channel have not been previously assign */
+                    if (channel_index >= 0) {
+                        if (s->pixel_type != EXR_UNKNOWN &&
+                            s->pixel_type != current_pixel_type) {
+                            av_log(s->avctx, AV_LOG_ERROR,
+                                   "RGB channels not of the same depth.\n");
+                            return AVERROR_INVALIDDATA;
+                        }
+                        s->pixel_type                     = current_pixel_type;
+                        s->channel_offsets[channel_index] = s->current_channel_offset;
                     }
-                    s->pixel_type                     = current_pixel_type;
-                    s->channel_offsets[channel_index] = s->current_channel_offset;
                 }
 
                 s->channels = av_realloc(s->channels,