Fix going back window no. on tabman close

[KSR-Yasuda]

In going back to the current active window on tabman close,
there is some case to move back to wrong window no.

Repro

" .vimrc

let g:tabman_side     = 'right'

" Open tabman (in right side) : tabman winnr = 2
" +---+-----------+
" |   |           |
" | 1 | 2: tabman |
" |   |           |
" +---+-----------+
\mt

" Open new window : winnr = 3
" +---+-----------+
" |   | 2: tabman |
" | 1 +-----------+
" |   |     3     |
" +---+-----------+
:below new

" Close tabman (winnr = 2),
" and try to go back to the window 3, but fails:
" the window number has been changed.
" +---+-----------+
" |   |           |
" | 1 |  3 -> 2   |
" |   |           |
" +---+-----------+
\mt