Fix oddly specific syntax error corrupting subsequent [[ ... ]]

Reproducer:

    $ x=([x]=1 [y)
    -ksh: syntax error: `)' unexpected
    $ [[ -z $x ]]
    -ksh: [[ -z  ]]: not found

Any '[[' command following that syntax error will fail similarly;
the whole of it (after variable expansion) is incorrectly looked up
as a command name. The syntax error must be generated by an
associative array assignment (with or without an explicit typeset
-A) with at least one valid assignment element followed by an
invalid assignment element starting with '[' but not containing
']='.

This seems to be another bug that is in every ksh93 version ever.
I've confirmed that ksh 1993-12-28 s+ and ksh2020 fail identically.
Presumably, so does everything in between.

Analysis:

The syntax error function, sh_syntax(), calls lexopen() in mode 0
to reset the lexer state. There is a variable that isn't getting
reset there though it should be. Using systematic elimination I
found that the variable that needs to be reset is lp->assignok (set
"when name=value is legal"). If it is set, '[[' is not processed.

src/cmd/ksh93/sh/lex.c: lexopen():
- Reset 'assignok' in the lexer state (regardless of mode).
- In the mode 0 total lexer state reinit, several members of lexd
  (struct _shlex_pvt_lexdata_) were not getting reset; just memset
  the whole thing to zero.
     Note for backporters: this change requires commit da97587e to
  be correct. That commit took the stack size and pointer (lex_max
  and *lex_match) out of this struct; those should not be reset!

Resolves: https://github.com/ksh93/ksh/issues/486

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-07-09:
+
+- Fixed a bug that broke '[[ ... ]]' test expressions for the command
+  following a syntax error in an associative array assignment.
+
 2022-07-05:
 
 - Fixed a spurious syntax error on encountering a process substitution