bug#12731: 24.2.50; dragging the mode-line selects other window

[martin rudalics]

> 1. C-x 2
> 2. C-x o
> 3. Drag the upper window's mode-line.
>
> The upper window is selected when you release mouse-1.  This makes no
> sense when I only wanted to enlarge the lower window.

The last line of `mouse-drag-line' should move into the preceding
`when' clause.

martin