avcodec/adpcm: Fix left shifts in AV_CODEC_ID_ADPCM_EA

Fixes: left shift of negative value -1
Fixes: 17683/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_EA_R2_fuzzer-5111690013704192

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 8695fbec57)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/adpcm.c

@@ -1287,10 +1287,10 @@ static int adpcm_decode_frame(AVCodecContext *avctx, void *data,
 
                     for (count2=0; count2<28; count2++) {
                         if (count2 & 1)
-                            next_sample = sign_extend(byte,    4) << shift;
+                            next_sample = (unsigned)sign_extend(byte,    4) << shift;
                         else {
                             byte = bytestream2_get_byte(&gb);
-                            next_sample = sign_extend(byte >> 4, 4) << shift;
+                            next_sample = (unsigned)sign_extend(byte >> 4, 4) << shift;
                         }
 
                         next_sample += (current_sample  * coeff1) +