[lldb] Only get child if m_storage and m_element_type is valid (llvm#163077)

This causes a crash because lldb-dap will check the first child to see
if it is array like to lazy load the children.

Author: da-viper

lldb/source/Plugins/Language/CPlusPlus/MsvcStlAtomic.cpp

@@ -50,7 +50,7 @@ llvm::Expected<uint32_t> lldb_private::formatters::
 lldb::ValueObjectSP
 lldb_private::formatters::MsvcStlAtomicSyntheticFrontEnd::GetChildAtIndex(
     uint32_t idx) {
-  if (idx == 0)
+  if (idx == 0 && m_storage && m_element_type.IsValid())
     return m_storage->Cast(m_element_type)->Clone(ConstString("Value"));
   return nullptr;
 }