avcodec/flacdsp_template: Fix undefined shift in flac_decorrelate_indep_c

Fixes: left shift of negative value
Fixes: 668346-media

Found-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit acc163c6ab)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/flacdsp_template.c

@@ -56,7 +56,7 @@ static void FUNC(flac_decorrelate_indep_c)(uint8_t **out, int32_t **in,
 
     for (j = 0; j < len; j++)
         for (i = 0; i < channels; i++)
-            S(samples, i, j) = in[i][j] << shift;
+            S(samples, i, j) = (int)((unsigned)in[i][j] << shift);
 }
 
 static void FUNC(flac_decorrelate_ls_c)(uint8_t **out, int32_t **in,