[ORC] Don't try to copy from an empty segment in SimpleExecutorMemoryManager.

Since 67220c2a empty SPSSequence<char>s deserialize to default-constructed
ArrayRef<char>s, which have a null data field. We need to check for this to
avoid memcpy'ing from a nullptr.

This should fix the bot failure in
https://lab.llvm.org/buildbot/#/builders/85/builds/9323