Vlexical_binding is sometimes NULL

[BT Templeton]

The `Vlexical_binding' is sometimes NULL in a context where it can be
accessed from Lisp code. For example, if I run `temacs -Q' (non-dumped
Emacs) and evaluate `lexical-binding' in the scratch buffer, it
evaluates to zero (NULL interpreted as a fixnum).

This doesn't cause any obvious problems right now, but it may if the
Lisp object representation changes; in Guile-Emacs it can cause
segfaults, for example. For now I've fixed it with the trivial patch
below, but would it be useful to add an initial-value argument to
DEFVAR_LISP to prevent this kind of problem in the future?

--- a/src/lread.c
+++ b/src/lread.c
@@ -4540,6 +4540,7 @@ with lexical binding.
 This variable is automatically set from the file variables of an
 interpreted Lisp file read using `load'.  Unlike other file local
 variables, this must be set in the first line of a file.  */);
+  Vlexical_binding = Qnil;
   Fmake_variable_buffer_local (Qlexical_binding);
 
   DEFVAR_LISP ("eval-buffer-list", Veval_buffer_list,

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