Fix request while protocol context is shutting down

[MartinHjelmare]

• If the protocol context is shutting down we're not allowed to make new requests with the context.
• Add a check if we're shutting down the protocol context when getting the context.
• This should fix reqeusts that are made after the protocol context has begun to shutdown but before the protocol context has completed the reset.
• Example stacktrace (first one): https://github.com/home-assistant/core/issues/105004#issuecomment-1855475069
• Requests that are already in flight before the protocol context begins the shutdown are up to aiocoap to handle and flush.