Fix vi mode crashes when going back one word (#246)

This bug was originally reported at <https://github.com/att/ast/issues/1467>.
A crash can occur when using the 'b' or 'B' vi mode commands to go back
one word. I was able to reproduce these crashes with 100% consistency on
an OpenBSD virtual machine when ksh is compiled with -D_std_malloc.
Reproducer:
    $ set -o vi
    $ asdf <ESC> <b or B>

The fix is based on Matthew DeVore's analysis:
> I suspect this is caused by this line:
>> while (vi_isalph(tcur_virt) && tcur_virt >= first_virt) --tcur_virt;
> which is in the b codepath. It checks vi_isalph(tcur_virt) before checking
> if tcur_virt is in range. These two clauses should be reversed. Note that
> line 316 is a similar check for pressing B, and there the tcur_virt value
> is checked first.

src/cmd/ksh93/edit/vi.c:
- Check tcur_virt before using isalph() or isblank() to fix both crashes.
  At the start of the backword() while loop this check was performed
  twice, so the redundant check has been removed.

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for the b, B, w and W editor commands.

src/cmd/ksh93/tests/pty.sh

@@ -770,5 +770,18 @@ w : XX\t
 r ^:test-1: : XXX\r\n$
 !
 
+((SHOPT_VSH)) && tst $LINENO <<"!"
+L Using b, B, w and W commands in vi mode
+# https://github.com/att/ast/issues/1467
+
+d 15
+p :test-1:
+w set -o vi
+r ^:test-1: set -o vi\r\n$
+w echo asdf\EbwBWa
+r ^:test-2: echo asdf\r\n$
+r ^asdf\r\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))