patch 8.2.2633: multi-byte 'fillchars' for folding do not show properly

Problem:    Multi-byte 'fillchars' for folding do not show properly.
Solution:   Handle multi-byte characters correctly. (Yegappan Lakshmanan,
            closes #7983, closes #7955)
diff --git a/src/screen.c b/src/screen.c
index 84951c5..d219b13 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -295,8 +295,13 @@
     if (closed)
     {
 	if (symbol != 0)
-	    // rollback length
+	{
+	    // rollback length and the character
 	    byte_counter -= len;
+	    if (len > 1)
+		// for a multibyte character, erase all the bytes
+		vim_memset(p + byte_counter, ' ', len);
+	}
 	symbol = fill_foldclosed;
 	len = utf_char2bytes(symbol, &p[byte_counter]);
 	byte_counter += len;