Fix data race on read/write _instance variable in ModuleData class

Summary:
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- [ ] Explain the **motivation** for making this change.
- [ ] Provide a **test plan** demonstrating that the code is solid.
- [ ] Match the **code formatting** of the rest of the codebase.
- [ ] Target the `master` branch, NOT a "stable" branch.

What existing problem does the pull request solve?
XCode  [Thread Sanitizer](https://clang.llvm.org/docs/ThreadSanitizer.html) find race condition while read/write `_instance` variable in RCTModuleData class. A bridge can check `hasInstance` method while instance writes.

All tests passed on my device.
These changes remove data race, you can turn it in scheme configuration ![](https://www.shinobicontrols.com/wp-content/uploads/2016/08/Enable_Sanitizer.png)
Closes #13757

Differential Revision: D4994041

Pulled By: javache

fbshipit-source-id: 631cd59bbcbde193937d8baf8358ff6868717a2e

React/Base/RCTModuleData.mm

@@ -214,6 +214,7 @@ - (void)setUpMethodQueue
 
 - (BOOL)hasInstance
 {
+  std::unique_lock<std::mutex> lock(_instanceLock);
   return _instance != nil;
 }