avoid mixing direct unlocking with lock-holders

Summary:
When using lock-holder objects, it is typically required to conduct all unlocking and relocking through them. That way, the RAII lock-holder does not attempt to unlock an unlocked mutex should the code throw after unlocking but before relocking.

Changelog: [internal]

Reviewed By: sammy-SC

Differential Revision: D29238721

fbshipit-source-id: 2403ce8c9e86f96b56a35a456e017486a2854c74

ReactCommon/react/renderer/componentregistry/ComponentDescriptorRegistry.cpp

@@ -60,9 +60,9 @@ ComponentDescriptor const &ComponentDescriptorRegistry::at(
 
   auto it = _registryByName.find(unifiedComponentName);
   if (it == _registryByName.end()) {
-    mutex_.unlock_shared();
+    lock.unlock();
     providerRegistry_.request(unifiedComponentName.c_str());
-    mutex_.lock_shared();
+    lock.lock();
 
     it = _registryByName.find(unifiedComponentName);