avfilter/vf_ciescope: Fix undefined behavior in rgb_to_xy() with black

Fixes: floating point division by 0
Fixes: undefined behavior in handling NaN
Fixes: Ticket 8268

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

libavfilter/vf_ciescope.c

@@ -849,7 +849,8 @@ rgb_to_xy(double rc,
     *z = m[2][0] * rc + m[2][1] * gc + m[2][2] * bc;
 
     sum = *x + *y + *z;
-
+    if (sum == 0)
+        sum = 1;
     *x = *x / sum;
     *y = *y / sum;
 }