mpeg12enc: use sign_extend() function

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/mpeg12enc.c

@@ -27,6 +27,7 @@
 
 #include "avcodec.h"
 #include "dsputil.h"
+#include "mathops.h"
 #include "mpegvideo.h"
 
 #include "mpeg12.h"
@@ -690,8 +691,7 @@ static void mpeg1_encode_motion(MpegEncContext *s, int val, int f_or_b_code)
         int bit_size = f_or_b_code - 1;
         int range = 1 << bit_size;
         /* modulo encoding */
-        int l= INT_BIT - 5 - bit_size;
-        val= (val<<l)>>l;
+        val = sign_extend(val, 5 + bit_size);
 
         if (val >= 0) {
             val--;