scale: set evaluated value for the "out_w" and "out_h" constants

The constant values were never set, thus the evaluation of expressions
referencing those constants were resulting in an error.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

libavfilter/vf_scale.c

@@ -165,12 +165,12 @@ static int config_props(AVFilterLink *outlink)
     av_expr_parse_and_eval(&res, (expr = scale->w_expr),
                            var_names, var_values,
                            NULL, NULL, NULL, NULL, NULL, 0, ctx);
-    scale->w = var_values[VAR_OW] = res;
+    scale->w = var_values[VAR_OUT_W] = var_values[VAR_OW] = res;
     if ((ret = av_expr_parse_and_eval(&res, (expr = scale->h_expr),
                                       var_names, var_values,
                                       NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
         goto fail;
-    scale->h = var_values[VAR_OH] = res;
+    scale->h = var_values[VAR_OUT_H] = var_values[VAR_OH] = res;
     /* evaluate again the width, as it may depend on the output height */
     if ((ret = av_expr_parse_and_eval(&res, (expr = scale->w_expr),
                                       var_names, var_values,