patch 8.0.1652: term_dumpwrite() does not output composing characters

Problem:    term_dumpwrite() does not output composing characters.
Solution:   Use the cell index.
diff --git a/src/terminal.c b/src/terminal.c
index 5875af6..b60799d 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3573,7 +3573,7 @@
 		    for (i = 0; i < VTERM_MAX_CHARS_PER_CELL
 						  && cell.chars[i] != NUL; ++i)
 		    {
-			len = utf_char2bytes(cell.chars[0], charbuf);
+			len = utf_char2bytes(cell.chars[i], charbuf);
 			fwrite(charbuf, len, 1, fd);
 		    }
 		}