palettegen: Fill with last color, not black

If we fill with black then the generated palette will have one color more
than what the user requested. This also resulted in unwanted black specks in
the output of paletteuse, especially when generating small palettes.

libavfilter/vf_palettegen.c

@@ -245,7 +245,7 @@ static void write_palette(AVFilterContext *ctx, AVFrame *out)
                     av_log(ctx, AV_LOG_WARNING, "Dupped color: %08"PRIX32"\n", pal[x]);
                 last_color = pal[x];
             } else {
-                pal[x] = 0xff000000; // pad with black
+                pal[x] = last_color; // pad with last color
             }
         }
         pal += pal_linesize;

tests/ref/fate/filter-palettegen-2

@@ -3,4 +3,4 @@
 #codec_id 0: rawvideo
 #dimensions 0: 16x16
 #sar 0: 1/1
-0,          0,          0,        1,     1024, 0x906ff5aa
+0,          0,          0,        1,     1024, 0x23e072c8