formatter: force the mark off, to not crash by accessing empty cutbuffer

More generally, after invoking the formatter, the entire buffer should
be cut (to be replaced with the reformatted text) and not just the part
before the mark, because then everything after it would be duplicated.

This fixes https://savannah.gnu.org/bugs/?58879.

Bug existed since version 4.9, commit 19517fc5.

src/text.c

@@ -2922,6 +2922,8 @@ void do_formatter(void)
 		return;
 	}
 
+	openfile->mark = NULL;
+
 	temp_name = safe_tempfile(&stream);
 
 	if (temp_name != NULL)