telnet_gui/screen: fix screen size TOCTOU

bounds check must be done inside the lock.. duh

telnet_gui/screen.go

@@ -220,11 +220,11 @@ func NewView(s *Screen, x, y, w, h int) (v *View) {
 }
 
 func (v *View) DrawBlockRaw(block []byte, x, y int) {
+	v.S.mutex.Lock()
+	defer v.S.mutex.Unlock()
 	if x < 0 || y < 0 || v.W <= x || v.H <= y {
 		return
 	}
-	v.S.mutex.Lock()
-	defer v.S.mutex.Unlock()
 	v.S.drawBlockUnlockedRaw(block, v.X + x, v.Y + y)
 }
 
@@ -233,11 +233,11 @@ func (v *View) DrawBlock(fg, bg Color, symbol byte, x, y int) {
 }
 
 func (v *View) DrawBlockAttr(fg, bg Color, symbol byte, x, y int, underline, bold Attr) {
+	v.S.mutex.Lock()
+	defer v.S.mutex.Unlock()
 	if x < 0 || y < 0 || v.W <= x || v.H <= y {
 		return
 	}
-	v.S.mutex.Lock()
-	defer v.S.mutex.Unlock()
 	v.S.drawBlockUnlocked(fg, bg, symbol, v.X+x, v.Y+y, underline, bold)
 }