Fix memory allocation in rclc_parameter

[bjsowa]

1. Fixed rclc_parameter_descriptor_initialize_string null dereference after failed allocation.
2. Simplified rclc_parameter_server_fini_memory function. There is no need to call fini on all fields of the message as the top-level fini function will do this either way.
3. Rewritten init functions to return on first error. This fixes issue with init_parameter_server_memory where it tries to dereference an object after it failed to allocate.

I haven't touched the low memory mode ~~but I'm fairly certain it can use the same fini memory function.~~