Fabric: Using non-mutating at instead of [] for `-[RCTSurfaceTouc…

…hHandler _activeTouches]`

Summary: Every C++ engineer (except me several months ago) knows that `operator []` can mutate the collection (Yeah! Don't ask), so this is especially dangerous if your hash function is broken (see the previous diff).

Reviewed By: mdvacca

Differential Revision: D13072805

fbshipit-source-id: 4436a8ff12fb27a57bfb6ee0ff986d7a9a825549

React/Fabric/RCTSurfaceTouchHandler.mm

@@ -196,14 +196,14 @@ - (void)_registerTouches:(NSSet<UITouch *> *)touches
 - (void)_updateTouches:(NSSet<UITouch *> *)touches
 {
   for (UITouch *touch in touches) {
-    UpdateActiveTouchWithUITouch(_activeTouches[touch], touch, _rootComponentView);
+    UpdateActiveTouchWithUITouch(_activeTouches.at(touch), touch, _rootComponentView);
   }
 }
 
 - (void)_unregisterTouches:(NSSet<UITouch *> *)touches
 {
   for (UITouch *touch in touches) {
-    const auto &activeTouch = _activeTouches[touch];
+    const auto &activeTouch = _activeTouches.at(touch);
     _identifierPool.enqueue(activeTouch.touch.identifier);
     _activeTouches.erase(touch);
   }