timing fix for RCTCxxBridge.executeApplicationScript (#25991)

Summary:
In one of our test apps (actually on Mac not iOS, but same code) we very consistently crash in RCTCxxBridge.executeApplicationScript when js debugging, due to a timing issue where another thread has reset _reactInstance in between the null check on self->_reactInstance and usage of it on these lines:

```
    } else if (self->_reactInstance) {
      self->_reactInstance->loadScriptFromString(std::make_unique<NSDataBigString>(script),

```
The thread doing the reset is doing so switching the executorClass to WebSocketExecutor.
In the scenario we crash, the packager has a bundle ready and quickly returns it, though its a 34MB string being passed to NSDataBigString which must be taking long enough for the other thread to get a chance to reset.

## Changelog

[iOS] [Fixed] - Fix crash in RCTCxxBridge.executeApplicationScript
Pull Request resolved: #25991

Test Plan: Ran apple code path in normal from bundle file and js debugging scenarios.

Reviewed By: shergin

Differential Revision: D19186065

Pulled By: hramos

fbshipit-source-id: ae1d4b5b50b7fb33b74aba21addc2978e917479f

Author: ahimberg

React/CxxBridge/RCTCxxBridge.mm

@@ -1334,19 +1334,23 @@ - (void)executeApplicationScript:(NSData *)script url:(NSURL *)url async:(BOOL)a
     [[NSNotificationCenter defaultCenter] postNotificationName:RCTJavaScriptWillStartExecutingNotification
                                                         object:self->_parentBridge
                                                       userInfo:@{@"bridge" : self}];
+    
+    // hold a local reference to reactInstance in case a parallel thread
+    // resets it between null check and usage
+    auto reactInstance = self->_reactInstance;
     if (isRAMBundle(script)) {
       [self->_performanceLogger markStartForTag:RCTPLRAMBundleLoad];
       auto ramBundle = std::make_unique<JSIndexedRAMBundle>(sourceUrlStr.UTF8String);
       std::unique_ptr<const JSBigString> scriptStr = ramBundle->getStartupCode();
       [self->_performanceLogger markStopForTag:RCTPLRAMBundleLoad];
       [self->_performanceLogger setValue:scriptStr->size() forTag:RCTPLRAMStartupCodeSize];
-      if (self->_reactInstance) {
+      if (reactInstance) {
         auto registry =
             RAMBundleRegistry::multipleBundlesRegistry(std::move(ramBundle), JSIndexedRAMBundle::buildFactory());
-        self->_reactInstance->loadRAMBundle(std::move(registry), std::move(scriptStr), sourceUrlStr.UTF8String, !async);
+        reactInstance->loadRAMBundle(std::move(registry), std::move(scriptStr), sourceUrlStr.UTF8String, !async);
       }
-    } else if (self->_reactInstance) {
-      self->_reactInstance->loadScriptFromString(
+    } else if (reactInstance) {
+      reactInstance->loadScriptFromString(
           std::make_unique<NSDataBigString>(script), sourceUrlStr.UTF8String, !async);
     } else {
       std::string methodName = async ? "loadApplicationScript" : "loadApplicationScriptSync";