message_get_thread_id: Generate internal error if message has no thread ID.

This case was happening when a message had its own message ID in its
In-Reply-To header. The thread-resolution code would find the
partially constructed message, (with no thread ID yet), get garbage
from this function, and then march right along with that garbage.

With this commit, a self-cyclic message like this will now trigger an
internal error rather than marching along silienty. (And a subsequent
commit will remove the call to this function in this case.)