patch 9.1.0113: duplicate code when cleaning undo stack

Problem:  duplicate code when cleaning undo stack
Solution: refactor undo cleanup into a single public function

related: #13928

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/fileio.c b/src/fileio.c
index 180fe39..d293d71 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4514,10 +4514,7 @@
 		// Mark the buffer as unmodified and free undo info.
 		unchanged(buf, TRUE, TRUE);
 		if ((flags & READ_KEEP_UNDO) == 0)
-		{
-		    u_blockfree(buf);
-		    u_clearall(buf);
-		}
+		    u_clearallandblockfree(buf);
 		else
 		{
 		    // Mark all undo states as changed.