8289002: Minimal x86_64 VM build fails with GCC 11: 'this' pointer is null

Reviewed-by: dholmes, aph

Author: shipilev

Diff for: src/hotspot/share/services/diagnosticFramework.cpp

@@ -462,6 +462,11 @@ void DCmdFactory::send_notification_internal(TRAPS) {
   if (notif) {
 
     Klass* k = Management::com_sun_management_internal_DiagnosticCommandImpl_klass(CHECK);
+    if (k == nullptr) {
+      fatal("Should have the DiagnosticCommandImpl class");
+      return; // silence the compiler
+    }
+
     InstanceKlass* dcmd_mbean_klass = InstanceKlass::cast(k);
 
     JavaValue result(T_OBJECT);

Diff for: src/hotspot/share/services/memoryManager.cpp

@@ -100,6 +100,11 @@ instanceOop MemoryManager::get_memory_manager_instance(TRAPS) {
       signature = vmSymbols::createMemoryManager_signature();
     }
 
+    if (k == nullptr) {
+      fatal("Should have the ManagementFactoryHelper or GarbageCollectorExtImpl class");
+      return nullptr; // silence the compiler
+    }
+
     InstanceKlass* ik = InstanceKlass::cast(k);
 
     JavaCalls::call_static(&result,