avcodec/huffyuvdec: Initialize whole output for decode_gray_bitstream()

Fixes: use of uninitialized memory
Fixes: 375286238/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HYMT_fuzzer-6352546854141952

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 ef71552cf970876085d99834abdb8e429aea9730)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/huffyuvdec.c

@@ -783,6 +783,8 @@ static void decode_gray_bitstream(HYuvDecContext *s, int count)
         for (i = 0; i < count && BITS_LEFT(re, &s->gb) > 0; i++) {
             READ_2PIX(s->temp[0][2 * i], s->temp[0][2 * i + 1], 0);
         }
+        for (; i < count; i++)
+            s->temp[0][2 * i] = s->temp[0][2 * i + 1] = 0;
     } else {
         for (i = 0; i < count; i++) {
             READ_2PIX(s->temp[0][2 * i], s->temp[0][2 * i + 1], 0);