lavc/alac: Make a variable unsigned.

Fixes a bogus compiler warning (max_samples_per_frame is checked):
libavcodec/alac.c: In function ‘allocate_buffers’:
./libavutil/internal.h:142:9: warning: argument 1 value ‘18446744073709551552’ exceeds maximum object size 9223372036854775807

libavcodec/alac.c

@@ -486,7 +486,7 @@ static av_cold int alac_decode_close(AVCodecContext *avctx)
 static int allocate_buffers(ALACContext *alac)
 {
     int ch;
-    int buf_size = alac->max_samples_per_frame * sizeof(int32_t);
+    unsigned buf_size = alac->max_samples_per_frame * sizeof(int32_t);
 
     for (ch = 0; ch < 2; ch++) {
         alac->predict_error_buffer[ch]  = NULL;