Lock the view while reading viewLines on mouse move

onMouseMove (and findHyperlinkAt, which it calls) read v.viewLines without
holding writeMutex, unlike every other reader. They run on the event-handling
goroutine, so a re-render on the task goroutine can shrink or rebuild viewLines
between onMouseMove's bounds check and findHyperlinkAt's indexing, causing an
out-of-range panic (observed: "index out of range [60] with length 0" while
hovering during a diff re-render).

Take writeMutex for the duration, like the other viewLines readers do, so the
check and the access see the same slice. Pre-existing, but the off-screen
re-render rebuilds viewLines on the task goroutine more often, widening the window.

pkg/gocui/view.go

@@ -2414,6 +2414,14 @@ func (v *View) onMouseMove(x int, y int) {
 		return
 	}
 
+	// Reading v.viewLines (here and in findHyperlinkAt) must hold writeMutex like
+	// every other reader: this runs on the event-handling goroutine, and a
+	// concurrent re-render on the task goroutine can rebuild or shrink viewLines
+	// between the bounds check below and the indexing in findHyperlinkAt — which
+	// panicked with an out-of-range index.
+	v.writeMutex.Lock()
+	defer v.writeMutex.Unlock()
+
 	// newCx and newCy are relative to the view port, i.e. to the visible area of the view
 	newCx := x - v.x0 - 1
 	newCy := y - v.y0 - 1