Don't execute some autocommands when v:dying is 2 or more.

runtime/doc/eval.txt

@@ -1348,7 +1348,9 @@ v:dying		Normally zero.	When a deadly signal is caught it's set to
 		terminate normally. {only works on Unix}
 		Example: >
 	:au VimLeave * if v:dying | echo "\nAAAAaaaarrrggghhhh!!!\n" | endif
-<
+<		Note: if another deadly signal is caught when v:dying is one,
+		VimLeave autocommands will not be executed.
+
 					*v:errmsg* *errmsg-variable*
 v:errmsg	Last given error message.  It's allowed to set this variable.
 		Example: >