Is it safe to connect a PoE injector switch to an ordinary PC?
There are many different manufacturers of network devices, and you didn't say what you are using, but I work with 24V PoE devices from Ubiquiti, so I'll answer based on those. If you are using something else, update the question and I'll update the answer.
No, it is not safe to plug an ordinary PC into a passively-powered 24V PoE switch output. Unlike standard 48V "active" PoE - i.e. 802.3af/at/bt - which checks whether the connected device needs to be powered, the "passive" PoE provided by 24V switches is always on if the port is configured to send power.
It is possible that some devices have current protection built into their network jacks, but that is definitely not something you want to rely on.
In theory, modern network jacks are required to be magnetically coupled instead of directly connected, specifically to prevent voltage spikes on the line from damaging the computer, but in practice this does not always seem to help for 24V Passive PoE.