avfilter/vf_nnedi: Fix use of uninitialized value

Fixes Coverity issue #1473546.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/vf_nnedi.c

@@ -903,7 +903,7 @@ static void subtract_mean_predictor(PredictorCoefficients *model)
 
     double softmax_means[256]; // Average of individual softmax filters.
     double elliott_means[256]; // Average of individual elliott filters.
-    double mean_filter[48 * 6]; // Pointwise average of all softmax filters.
+    double mean_filter[48 * 6] = { 0 }; // Pointwise average of all softmax filters.
     double mean_bias;
 
     // Quality 1.