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

src/cmd/ksh93/sh/parse.c

@@ -1507,7 +1507,7 @@ static Shnode_t *simple(Lex_t *lexp,int flag, struct ionod *io)
 			lexp->token = LBRACE;
 			break;
 		}
-		if(associative && argp->argval[0]!='[')
+		if(associative && (argp->argval[0]!='[' || !strstr(argp->argval,"]=")))
 			sh_syntax(lexp);
 		/* check for assignment argument */
 		if((argp->argflag&ARG_ASSIGN) && assignment!=2)