Invoking devMenu from RCTBridge in prod will crash

Summary: While we shouldn't invoke `bridge.devMenu` in production, doing it will result into a crash.

1. `devMenu` internally calls `RCTBridgeModuleNameForClass([RCTDevMenu class])`
2. `RCTBridgeModuleNameForClass()` calls `moduleName`
3. In the release code `RCTDevMenu` doesn't export the `moduleName` class method.
Closes https://github.com/facebook/react-native/pull/2910

Reviewed By: svcscm

Differential Revision: D2550797

Pulled By: tadeuzagallo

fb-gh-sync-id: 5dfbf905e5a02d9fd3b52f8b3d6eefc4e3ff30b2

React/Modules/RCTDevMenu.m

@@ -623,7 +623,11 @@ RCT_EXPORT_METHOD(reload)
 
 - (RCTDevMenu *)devMenu
 {
+#if RCT_DEV
   return self.modules[RCTBridgeModuleNameForClass([RCTDevMenu class])];
+#else
+  return nil;
+#endif
 }
 
 @end