bug#15786: 24.3; No warning on mal-formed let form when lexical-binding

[Leo Liu]

Due to a typo in my code, I got a bug report on ggtags
https://github.com/leoliu/ggtags/issues/17. I was fooled by the elisp
compiler which warns mal-formed let forms without lexical-binding.

To reproduce
1. Download the file in the attachment
2. byte-compile it
3. remove the first line and byte-compile it

step 2 produces no warnings while step 3 does.

t.el
Description: application/emacs-lisp