Added lock around RN module initialization to fix crash

Summary:
T35879909 is a UBN caused by a race condition in RN initialization. I haven't been able to repro, but the crash logs point to a bad memory access in this method. Another thread must be deallocating something concurrently.

This is a quick fix to patch into v197.

Reviewed By: fkgozali

Differential Revision: D12904277

fbshipit-source-id: 5740183f9a7c8f2c45ca627662891cb0c1048764

React/CxxBridge/RCTCxxBridge.mm

@@ -706,9 +706,11 @@ struct RCTInstanceCallback : public InstanceCallback {
 
 - (void)registerAdditionalModuleClasses:(NSArray<Class> *)modules
 {
-  NSArray<RCTModuleData *> *newModules = [self _initializeModules:modules withDispatchGroup:NULL lazilyDiscovered:YES];
-  if (_reactInstance) {
-    _reactInstance->getModuleRegistry().registerModules(createNativeModules(newModules, self, _reactInstance));
+  @synchronized (self) {
+    NSArray<RCTModuleData *> *newModules = [self _initializeModules:modules withDispatchGroup:NULL lazilyDiscovered:YES];
+    if (_reactInstance) {
+      _reactInstance->getModuleRegistry().registerModules(createNativeModules(newModules, self, _reactInstance));
+    }
   }
 }