avcodec/dirac_vlc: Fix integer overflow in ff_dirac_golomb_read_32/16bit()

Fixes: left shift of 1073741824 by 1 places cannot be represented in type 'int32_t' (aka 'int')
Fixes: 21245/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5683334274613248

Change to int16_t suggested by Lynne

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

libavcodec/dirac_vlc.c

@@ -1095,7 +1095,7 @@ int ff_dirac_golomb_read_16bit(const uint8_t *buf, int bytes,
 {
     LUTState lut = ff_dirac_golomb_lut[*buf++];
     int16_t *dst = (int16_t *)_dst, *last = dst + coeffs;
-    int16_t val = 0;
+    uint16_t val = 0;
 
     for (int i = 1; i < bytes; i++)
         PROCESS_VALS;
@@ -1115,7 +1115,7 @@ int ff_dirac_golomb_read_32bit(const uint8_t *buf, int bytes,
 {
     LUTState lut = ff_dirac_golomb_lut[*buf++];
     int32_t *dst = (int32_t *)_dst, *last = dst + coeffs;
-    int32_t val = 0;
+    uint32_t val = 0;
 
     for (int i = 1; i < bytes; i++)
         PROCESS_VALS;