Prevent mouse interaction crashing app

Possibly helps everyone in #63, #73 and #94

Whether it solves the issue or not, this definitely fixes a bug.
`CurrentTab` doesn't refer to anything until the first frame is received
from the webextension, yet tthe `handleMouseEvent` function can be
triggered long before that.

interfacer/src/browsh/tty.go

@@ -147,6 +147,7 @@ func handleScrolling(ev *tcell.EventKey) {
 }
 
 func handleMouseEvent(ev *tcell.EventMouse) {
+	if CurrentTab == nil { return }
 	x, y := ev.Position()
 	xInFrame := x + CurrentTab.frame.xScroll
 	yInFrame := y - uiHeight + CurrentTab.frame.yScroll