avcodec/ilbcdec: Initialize tempbuff2

Fixes: Use of uninitialized value
Fixes: 71350/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ILBC_fuzzer-6322020827070464

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

libavcodec/ilbcdec.c

@@ -658,7 +658,7 @@ static void get_codebook(int16_t * cbvec,   /* (o) Constructed codebook vector *
     int16_t k, base_size;
     int16_t lag;
     /* Stack based */
-    int16_t tempbuff2[SUBL + 5];
+    int16_t tempbuff2[SUBL + 5] = {0};
 
     /* Determine size of codebook sections */
     base_size = lMem - cbveclen + 1;