Fix kore_block_dump

[tothtamas28]

The previous implementation called std::strlen on the data field of the string * returned by print_configuration_to_string, then used that length to allocate and copy the result. However, the runtime string type is not null terminated, strlen therefore reads past the end of the buffer.

This is fixed by calling len() on the string * to obtain the correct length, and replacing strncpy with memcpy to copy exactly that many bytes.