patch 9.0.0774: the libvterm code is outdated

Problem:    The libvterm code is outdated.
Solution:   Include libvterm changes from revision 802 to 817.  Revert some
            changes made for C89.
diff --git a/src/terminal.c b/src/terminal.c
index d9f1e0f..1da9d2d 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3447,7 +3447,8 @@
   handle_bell,		// bell
   handle_resize,	// resize
   handle_pushline,	// sb_pushline
-  NULL			// sb_popline
+  NULL,			// sb_popline
+  NULL			// sb_clear
 };
 
 /*