avcodec/apedec: Only clear the needed buffer space, instead of all

Fixes: Timeout (15sec -> 0.4sec)
Fixes: 18396/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APE_fuzzer-5730080487112704

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/apedec.c

@@ -1516,7 +1516,7 @@ static int ape_decode_frame(AVCodecContext *avctx, void *data,
     av_fast_malloc(&s->decoded_buffer, &s->decoded_size, decoded_buffer_size);
     if (!s->decoded_buffer)
         return AVERROR(ENOMEM);
-    memset(s->decoded_buffer, 0, s->decoded_size);
+    memset(s->decoded_buffer, 0, decoded_buffer_size);
     s->decoded[0] = s->decoded_buffer;
     s->decoded[1] = s->decoded_buffer + FFALIGN(blockstodecode, 8);