RTPS: Fixup our coherent set map.
coherent_set_tracking.coherent_set_registry_map uses a struct as a key, but the hash and comparison routines treat keys as a sequence of bytes. Make sure every key byte is initialized. Fixes #16994. Call wmem_strong_hash on our key in coherent_set_key_hash_by_key instead of creating and leaking a GBytes struct.