avfilter/vf_fieldhint: Reorder operation to prevent hypothetical integer overflow

Fixes CID1355110
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_fieldhint.c

@@ -174,8 +174,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
 
     switch (s->mode) {
     case 0:
-        top    = s->frame[1 + tf - outlink->frame_count];
-        bottom = s->frame[1 + bf - outlink->frame_count];
+        top    = s->frame[tf - outlink->frame_count + 1];
+        bottom = s->frame[bf - outlink->frame_count + 1];
         break;
     case 1:
         top    = s->frame[1 + tf];