avcodec/dvbsubdec: Check entry_id

Fixes: randomly writing over the array end
Fixes: 1473/clusterfuzz-testcase-minimized-5768907824562176

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

libavcodec/dvbsubdec.c

@@ -1103,9 +1103,9 @@ static int dvbsub_parse_clut_segment(AVCodecContext *avctx,
                 return AVERROR_INVALIDDATA;
         }
 
-        if (depth & 0x80)
+        if (depth & 0x80 && entry_id < 4)
             clut->clut4[entry_id] = RGBA(r,g,b,255 - alpha);
-        else if (depth & 0x40)
+        else if (depth & 0x40 && entry_id < 16)
             clut->clut16[entry_id] = RGBA(r,g,b,255 - alpha);
         else if (depth & 0x20)
             clut->clut256[entry_id] = RGBA(r,g,b,255 - alpha);