FIRE-34497 - Bugsplat - crash during shutdown due to null mutex.

This is evidently a race condition, not everyone is crashing and according to LL it does not repro in DeltaFPS (though it does in Atlasaurus for them too). This probably suggests that DeltaFPS just happened to move the race condition needle a little in the right direction.

indra/llplugin/llpluginprocessparent.cpp

@@ -187,8 +187,18 @@ LLPluginProcessParent::ptr_t LLPluginProcessParent::create(LLPluginProcessParent
 /*static*/
 void LLPluginProcessParent::shutdown()
 {
-    LLCoros::LockType lock(*sInstancesMutex);
-
+    // <FS:Beq> FIRE-34497 - lock maybe be null during shutdown due to fiber shutdown race condition
+    // LLCoros::LockType lock(*sInstancesMutex);
+    std::unique_ptr<LLCoros::LockType> lock;
+    if (sInstancesMutex)
+    {
+        lock = std::make_unique<LLCoros::LockType>(*sInstancesMutex);
+    }
+    else
+    {
+        LL_WARNS("Plugin") << "shutdown called but no instances mutex available" << LL_ENDL;
+    }
+    // </FS:Beq>
     mapInstances_t::iterator it;
     for (it = sInstances.begin(); it != sInstances.end(); ++it)
     {