yadif: correct strides in filter_edges_16bit

The C code treats the data as arrays of uint16_t so strides must not
be in bytes but in pixels.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_yadif.c

@@ -158,6 +158,8 @@ static void filter_edges_16bit(void *dst1, void *prev1, void *cur1, void *next1,
     int x;
     uint16_t *prev2 = parity ? prev : cur ;
     uint16_t *next2 = parity ? cur  : next;
+    mrefs /= 2;
+    prefs /= 2;
 
     FILTER(0, 3, 0)