avcodec/exr: Check oe in huf_decode() before use

Fixes: out of array access
Fixes: 31386/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-5773234709594112

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

@@ -422,7 +422,12 @@ static int huf_decode(VLC *vlc, GetByteContext *gb, int nbits, int run_sym,
 
         if (x == run_sym) {
             int run = get_bits(&gbit, 8);
-            uint16_t fill = out[oe - 1];
+            uint16_t fill;
+
+            if (oe == 0 || oe + run > no)
+                return AVERROR_INVALIDDATA;
+
+            fill = out[oe - 1];
 
             while (run-- > 0)
                 out[oe++] = fill;