avfilter/vf_signature: Fix integer overflow in filter_frame()

Fixes: CID1403233

The second of the 2 changes may be unneeded but will help coverity

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

libavfilter/vf_signature.c

@@ -219,7 +219,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *picref)
     dw1 = inlink->w / 32;
     if (inlink->w % 32)
         dw2 = dw1 + 1;
-    denom = (sc->divide) ? dh1 * dh2 * dw1 * dw2 : 1;
+    denom = (sc->divide) ? dh1 * (int64_t)dh2 * dw1 * dw2 : 1;
 
     for (i = 0; i < 32; i++) {
         rowcount = 0;
@@ -245,7 +245,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *picref)
         }
     }
 
-    denom = (sc->divide) ? 1 : dh1 * dh2 * dw1 * dw2;
+    denom = (sc->divide) ? 1 : dh1 * (int64_t)dh2 * dw1 * dw2;
 
     for (i = 0; i < ELEMENT_COUNT; i++) {
         const ElemCat* elemcat = elements[i];