LogBox - Deallocate _rootViewController to release surface reference and break cycle

Summary:
This def breaks a reference cycle in logbox causing a memory leak in development.

Changelog: [iOS] [Fix] LogBox - Fix dependency cycle

Reviewed By: PeteTheHeat

Differential Revision: D19768068

fbshipit-source-id: 518b9c66499aa092465a9213f955965bffeebd88

Author: rickhanlonii

React/CoreModules/RCTLogBox.mm

@@ -70,8 +70,12 @@ - (void)dealloc
 
 - (void)show
 {
+  __weak __typeof(self) weakSelf = self;
   [RCTSharedApplication().delegate.window.rootViewController presentViewController:_rootViewController animated:NO completion:^{
-    [self->_rootViewController.view becomeFirstResponder];
+    __strong __typeof(self) strongSelf = weakSelf;
+    if (strongSelf) {
+      [strongSelf->_rootViewController.view becomeFirstResponder];
+    }
   }];
 }