avcodec/exr: Check buf_size more completely

Fixes: Out of heap array read
Fixes: 4683/clusterfuzz-testcase-minimized-6152313673613312

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

libavcodec/exr.c

@@ -1051,7 +1051,7 @@ static int decode_block(AVCodecContext *avctx, void *tdata,
     line_offset = AV_RL64(s->gb.buffer + jobnr * 8);
 
     if (s->is_tile) {
-        if (line_offset > buf_size - 20)
+        if (buf_size < 20 || line_offset > buf_size - 20)
             return AVERROR_INVALIDDATA;
 
         src  = buf + line_offset + 20;
@@ -1062,7 +1062,7 @@ static int decode_block(AVCodecContext *avctx, void *tdata,
         tile_level_y = AV_RL32(src - 8);
 
         data_size = AV_RL32(src - 4);
-        if (data_size <= 0 || data_size > buf_size)
+        if (data_size <= 0 || data_size > buf_size - line_offset - 20)
             return AVERROR_INVALIDDATA;
 
         if (tile_level_x || tile_level_y) { /* tile level, is not the full res level */
@@ -1095,7 +1095,7 @@ static int decode_block(AVCodecContext *avctx, void *tdata,
         td->channel_line_size = td->xsize * s->current_channel_offset;/* uncompress size of one line */
         uncompressed_size = td->channel_line_size * (uint64_t)td->ysize;/* uncompress size of the block */
     } else {
-        if (line_offset > buf_size - 8)
+        if (buf_size < 8 || line_offset > buf_size - 8)
             return AVERROR_INVALIDDATA;
 
         src  = buf + line_offset + 8;
@@ -1105,7 +1105,7 @@ static int decode_block(AVCodecContext *avctx, void *tdata,
             return AVERROR_INVALIDDATA;
 
         data_size = AV_RL32(src - 4);
-        if (data_size <= 0 || data_size > buf_size)
+        if (data_size <= 0 || data_size > buf_size - line_offset - 8)
             return AVERROR_INVALIDDATA;
 
         td->ysize          = FFMIN(s->scan_lines_per_block, s->ymax - line + 1); /* s->ydelta - line ?? */