Further fix '<>;' and fix crash on 32-bit systems (re: 6701bb30)

Accessing t->tre.treio for every sh_exec() run is invalid because
't' is of type Shnode_t, which is a union that can contain many
different kinds of structs. As all members of a union occupy the
same address space, only one can be used at a time. Which member is
valid to access depends on the node type sh_exec() was called with.
The invalid access triggered a crash on 32-bit systems when
executing an arithmetic command like ((x=1)).

The t->tre.treio union member should be accessed for a simple
command (case TCOM in sh_exec()). The fix is also needed for
redirections attached to blocks (case TSETIO) in which case the
union member to use is t->fork.forkio.

src/cmd/ksh93/sh/xec.c:
- Add check_exec_optimization() function that checks for all the
  conditions where the exec optimisation should not be done. For
  redirections we need to loop through the whole list to check for
  an IOREWRITE (<>;) one.
- sh_exec(): case TCOM (simple command): Only bother to call
  check_exec_optimization() if there are either command arguments
  or redirections (IOW: don't bother for bare variable
  assignments), so move it to within the if(io||argn) block.
- sh_exec(): case TSETIO: This needs a similar fix. To avoid the
  optimization breaking again if the last command is a subshell
  with a <>; redirection attached, we need to not only set execflg
  to 0 but also clear the SH_NOFORK state bit from the 'flags'
  variable which is passed on to the recursive sh_exec() call.

src/cmd/ksh93/tests/io.sh:
- Update and expand tests. Add tests for redirections attached to
  simple commands (TCOM) and various kinds of code block (TSETIO).

Co-authored-by: Johnothan King <johnothanking@protonmail.com>
Resolves: https://github.com/ksh93/ksh/issues/278

NEWS

@@ -8,10 +8,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed a bug in emacs mode: after using tab completion to complete the name
   of a directory, it was not possible to type numbers after the slash.
 
-2021-04-15:
-
 - Fixed an optimization bug that caused the <>; redirection operator to fail
-  when used in -c scripts.
+  when used with the last command in a -c script.
 
 2021-04-14: