avcodec/diracdec: Fix integer overflow with quant

Fixes: signed integer overflow: 2 + 2147483646 cannot be represented in type 'int'
Fixes: 4792/clusterfuzz-testcase-minimized-6322450775146496

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit eaa9317589)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/diracdec.c

@@ -532,16 +532,16 @@ static inline void codeblock(DiracContext *s, SubBand *b,
     }
 
     if (s->codeblock_mode && !(s->old_delta_quant && blockcnt_one)) {
-        int quant = b->quant;
+        int quant;
         if (is_arith)
-            quant += dirac_get_arith_int(c, CTX_DELTA_Q_F, CTX_DELTA_Q_DATA);
+            quant = dirac_get_arith_int(c, CTX_DELTA_Q_F, CTX_DELTA_Q_DATA);
         else
-            quant += dirac_get_se_golomb(gb);
-        if (quant < 0) {
+            quant = dirac_get_se_golomb(gb);
+        if (quant > INT_MAX - b->quant || b->quant + quant < 0) {
             av_log(s->avctx, AV_LOG_ERROR, "Invalid quant\n");
             return;
         }
-        b->quant = quant;
+        b->quant += quant;
     }
 
     b->quant = FFMIN(b->quant, MAX_QUANT);