Fix vi mode tab completion with spaces (#413)

Attempting to complete file names in vi mode using tab completion can
fail if the last character on the command line is a space. Reproducer
(note that this bug doesn't occur in emacs mode):
   $ set -o vi
   $ mkdir '/tmp/foo bar'
   $ test -d /tmp/foo\ <Tab>

src/cmd/ksh93/edit/vi.c:
- Don't disable tab completion or reset the tab count just because the
  last character on the command line is a space. This bugfix was
  backported from ksh93v- 2014-06-06.

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for the tab completion bug.

src/cmd/ksh93/tests/pty.sh

@@ -946,5 +946,17 @@ u ^OK :child-!: \r\n$
 w exit
 !
 
+touch "$tmp/foo bar"
+((SHOPT_VSH || SHOPT_ESH)) && tst $LINENO <<!
+L tab completion with space in string
+# https://github.com/ksh93/ksh/pull/413
+
+d 15
+p :test-1:
+w echo $tmp/foo\\\\ \\t
+r ^:test-1: echo $tmp/foo\\\\ bar \\r\\n$
+r ^$tmp/foo bar\\r\\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))