single provider fetching of ad chain fails with multihash too short error

[masih]

The Index Provider link system generates the entry chunks on the fly if they are not cached. To do this, it looks up the mapping of a given cid to its context ID and provider via this function. Later it converts the returned provider byte slice to peer ID to then call the multihash lister for getting the list of entries.

The issue is that getCidKeyMap can return nil slice as provider ID which then fails to be converted into peer ID since it is empty which then results in multihash too short error.

To fix this getCidKeyMap must guarantee that it never returns nil Provider.

• In the case of legacy it should return the engine's default provider.
• In the case of non-legacy it should still sanity check that provider is not empty and fill it with the default provider if it is.