avfilter/vf_readeia608: if parity bit check fails, set correct value

As described in U.S. Federal Register, Volume 56, Number 114, June 13, 1991, pages 27204-27205.

libavfilter/vf_readeia608.c

@@ -365,7 +365,7 @@ static void extract_line(AVFilterContext *ctx, AVFilterLink *inlink, AVFrame *in
 
         if (s->chp) {
             if (!(parity & 1)) {
-                byte[ch] = 0;
+                byte[ch] = 0x7F;
             }
         }
     }