If the adapter comes with the adapter it should be safe, the kit is designed for both desktop and laptop hdd, and desktop HDD draws power mostly from 12V so there is a 12V output for the adapter. And you wouldn't get 12V on the 5V input if the power supply works properly.
However, I suggest you to just hook the laptop HDD to desktop directly, if the desktop has spare SATA power plug and SATA cable available, you can plugin and use the laptop hdd just like a normal desktop HDD, in this way its faster and usually more reliable.