4xm: fix signed overflow

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 84dda40762)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/4xm.c

@@ -277,7 +277,7 @@ static void init_mv(FourXContext *f){
     }
 #endif
 
-static inline void mcdc(uint16_t *dst, uint16_t *src, int log2w, int h, int stride, int scale, int dc){
+static inline void mcdc(uint16_t *dst, uint16_t *src, int log2w, int h, int stride, int scale, unsigned dc){
    int i;
    dc*= 0x10001;