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/netbeans.c b/src/netbeans.c
index ebdf3a4..d542b58 100644
--- a/src/netbeans.c
+++ b/src/netbeans.c
@@ -1285,8 +1285,7 @@
 		netbeansFireChanges = oldFire;
 		netbeansSuppressNoLines = oldSuppress;
 
-		u_blockfree(buf->bufp);
-		u_clearall(buf->bufp);
+		u_clearallandblockfree(buf->bufp);
 	    }
 	    nb_reply_nil(cmdno);
 // =====================================================================
@@ -1456,8 +1455,7 @@
 		netbeansFireChanges = oldFire;
 
 		// Undo info is invalid now...
-		u_blockfree(curbuf);
-		u_clearall(curbuf);
+		u_clearallandblockfree(curbuf);
 	    }
 	    vim_free(to_free);
 	    nb_reply_nil(cmdno); // or !error