bug#24641: 26.0.50; let-alist: Incorrect byte-compile warning about unused lexical variable

[Philipp <p.stephani2@gmail.com>]

$ cat /tmp/letalist.el 
;; -*- lexical-binding: t; -*-
(print
 (let-alist '((outer . ((inner . value))))
   (let-alist .outer .inner)))
   
$ emacs -Q -batch -f batch-byte-compile /tmp/letalist.el 

In toplevel form:
/tmp/letalist.el:2:1:Warning: Unused lexical variable ‘\.inner’

This warning is incorrect because '.inner' is used in the inner
let-alist form.


In GNU Emacs 26.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2016-10-07 built on localhost
Repository revision: 1686a0cde3d6adced3b5393945d6a9ab71b4a3c9
Windowing system distributor 'The X.Org Foundation', version 11.0.11702000
System Description:	Ubuntu 14.04 LTS

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Configured features:
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Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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  global-font-lock-mode: t
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  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
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term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
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Memory information:
((conses 16 97276 8627)
 (symbols 48 20242 0)
 (miscs 40 330 118)
 (strings 32 17820 3260)
 (string-bytes 1 578593)
 (vectors 16 13702)
 (vector-slots 8 448689 6242)
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 (buffers 976 11)
 (heap 1024 46517 1112))