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/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 903be5e4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/exr.c

@@ -846,7 +846,7 @@ static int decode_block(AVCodecContext *avctx, void *tdata,
 
     line_offset = AV_RL64(s->gb.buffer + jobnr * 8);
     // Check if the buffer has the required bytes needed from the offset
-    if (line_offset > buf_size - 8)
+    if (buf_size < 8 || line_offset > buf_size - 8)
         return AVERROR_INVALIDDATA;
 
     src  = buf + line_offset + 8;
@@ -855,7 +855,7 @@ 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;
 
     s->ysize          = FFMIN(s->scan_lines_per_block, s->ymax - line + 1);