null-terminate registry value in windows_get_device_registry_entry

[aizu-m]

1. RegQueryValueEx sizes manifest_path to the value length, but its docs warn REG_SZ/REG_MULTI_SZ values may not be stored with a terminating null.
2. windows_add_json_entry then walks that buffer with strlen, so an unterminated value reads past the allocation.

Reserved two bytes and null-terminated the value after the read, as the RegQueryValueEx docs recommend.

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