alacenc: Fix missing sign_extend()

Possibly fixes Ticket2497
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/alacenc.c

@@ -274,7 +274,7 @@ static void alac_linear_predictor(AlacEncodeContext *s, int ch)
         // generate warm-up samples
         residual[0] = samples[0];
         for (i = 1; i <= lpc.lpc_order; i++)
-            residual[i] = samples[i] - samples[i-1];
+            residual[i] = sign_extend(samples[i] - samples[i-1], s->write_sample_size);
 
         // perform lpc on remaining samples
         for (i = lpc.lpc_order + 1; i < s->frame_size; i++) {