tools/target_dec_fuzzer: Only audio uses the return value to decode packets in pieces, correct the code to match that

Fixes: Timeout
Fixes: 483/fuzz-0-ffmpeg_VIDEO_AV_CODEC_ID_H263I_fuzzer

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

tools/target_dec_fuzzer.c

@@ -186,6 +186,8 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 
             if (ret <= 0 || ret > avpkt.size)
                break;
+            if (ctx->codec_type != AVMEDIA_TYPE_AUDIO)
+                ret = avpkt.size;
             avpkt.data += ret;
             avpkt.size -= ret;
         }