Fix syntax error detection in associative array assignments

Reproducer:

$ fn=([foo_key]=foo_val [bar_key])
-ksh: [bar_key]: not found

Expected output:
-ksh: syntax error: `[bar_key]' unexpected

As soon as one correct associative array assignment element has
been processed, a subsequent one, starting with '[' but not
containing ']=', is incorrectly seen as a command to execute.
If a command '[bar_key]' existed on $PATH, it would have been run.

src/cmd/ksh93/sh/parse.c: simple():
- In the syntax check for associative array assignments, don't just
  check for an initial '[' but also verify the presence of ']='.

Thanks to @JohnoKing for finding this bug.

Resolves: https://github.com/ksh93/ksh/issues/427

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   following a redirection without being an argument to a redirection.
   For example, this now writes 'OK' to standard error: cat >&2 <(echo OK)
 
+- Fixed the detection of a syntax error in compound assignments to
+  associative arays.
+
 2022-07-02:
 
 - Fixed a bug where, if the last command in a subshell was an external