tui: fix uninitialize variable usage (#8261)

This fixes an issue with compiles failing in release mode due to shape
having the possibility of being used uninitialized (since the default
case was missing).

src/nvim/tui/tui.c

@@ -937,6 +937,7 @@ static void tui_set_mode(UI *ui, ModeShape mode)
 
   int shape;
   switch (c.shape) {
+    default:
     case SHAPE_BLOCK: shape = 1; break;
     case SHAPE_HOR:   shape = 3; break;
     case SHAPE_VER:   shape = 5; break;