patch 8.0.0524: folds messed up

Problem:    Folds are messed up when 'encodin' is "utf-8".
Solution:   Also set the fold character when it's not multi-byte.
diff --git a/src/screen.c b/src/screen.c
index ceb30e3..fbfded1 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2700,7 +2700,10 @@
                 ScreenLines[off + col] = 0x80; /* avoid storing zero */
 	    }
 	    else
+	    {
 		ScreenLinesUC[off + col] = 0;
+		ScreenLines[off + col] = fill_fold;
+	    }
 	    col++;
 	}
 	else