avcodec/adpcm: Fix undefined behavior with negative predictions in IMA OKI

Fixes: left shift of negative value -30
Fixes: 18392/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_IMA_OKI_fuzzer-5631771831435264

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

libavcodec/adpcm.c

@@ -342,7 +342,7 @@ static inline int16_t adpcm_ima_oki_expand_nibble(ADPCMChannelStatus *c, int nib
     c->predictor = av_clip_intp2(predictor, 11);
     c->step_index = step_index;
 
-    return c->predictor << 4;
+    return c->predictor * 16;
 }
 
 static inline int16_t adpcm_ct_expand_nibble(ADPCMChannelStatus *c, int8_t nibble)