Fix command history corruption on syntax error (re: e999f6b1)

Analysis: When a syntax error occurs, the shell performs a
longjmp(3) back to exfile() in main.c on line 417:
415|	if(jmpval)
416|	{
417|		Sfio_t *top;
418|		sh_iorestore((void*)shp,0,jmpval);
419|		hist_flush(shp->gd->hist_ptr);
420|		sfsync(shp->outpool);
The first thing it does is restore the file descriptor state
(sh_iorestore), then it flushes the history file (hist_flush), then
it synchronises sfio's logical stream state with the physical
stream state using (sfsync).

However, the fix applied in e999f6b1 caused sh_iorestore() to sync
all sfio streams unconditionally. So this was done before
hist_flush(), which caused unpredictable behaviour, including
temporary and/or permanent history corruption, as this also synched
shp->outpool before hist_flush() had a chance to do its thing.

The fix is to only call sfsync() in sh_iorestore() if we're
actually about to call ftruncate(2), and not otherwise.

Moral of the story: bug fixes should be as specific as possible to
minimise the risk of side effects.

src/cmd/ksh93/sh/io.c: sh_iorestore():
- Only call sfsync() if we're about to truncate a file.

src/cmd/ksh93/tests/pty.sh:
- Add test.

Thanks to Marc Wilson for reporting the bug and to Johnothan King
for finding the commit that introduced it.

Resolves: https://github.com/ksh93/ksh/issues/209
Relevant: https://github.com/att/ast/issues/61

src/cmd/ksh93/tests/pty.sh

@@ -107,7 +107,6 @@ if	! pty $bintrue < /dev/null
 then	err_exit pty command hangs on $bintrue -- tests skipped
 	exit 0
 fi
-
 # err_exit #
 tst $LINENO <<"!"
 L POSIX sh 026(C)
@@ -712,5 +711,21 @@ r ^:test-3: ls vi_completion_A_file\r\n$
 r ^vi_completion_A_file\r\n$
 !
 
+# err_exit #
+tst $LINENO <<"!"
+L syntax error added to history file
+
+# https://github.com/ksh93/ksh/issues/209
+
+d 10
+p :test-1:
+w do something
+r ^:test-1: do something\r\n$
+r : syntax error: `do' unexpected\r\n$
+w fc -lN1
+r ^:test-2: fc -lN1\r\n$
+r \tdo something\r\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))