wezterm: fix resize crash

Repro steps were:

* Split horizontal
* Resize y to be shorter; size shrinks to zero
* Resize y to be taller; size grows to u16::max and then overflows

The return statement was missing; probably a copy-n-paste-o as
the equivalent logic for the x size adjustment had it.

closes: https://github.com/wez/wezterm/issues/276

src/mux/tab.rs

@@ -412,6 +412,7 @@ fn adjust_y_size(tree: &mut Tree, mut y_adjust: isize, cell_dimensions: &PtySize
                         data.second.rows = data.first.rows;
                         data.second.pixel_height = data.first.pixel_height;
                     }
+                    return;
                 }
                 SplitDirection::Vertical if y_adjust > 0 => {
                     adjust_y_size(&mut *left, 1, cell_dimensions);