Most newer flashlights have over-discharge protection, and most flashlights with built-in charging terminate properly without overcharging cells. Both are true of the FC11, and its original battery is unprotected. It is a very popular flashlight, and I have never seen a post about a safety-related malfunction with one in any of the popular flashlight communities.

@dr_jekell@lemmy.world has a point that cells salvaged from random low-quality devices may be unsuitable for another application (or even their original application). There's a chance that the current draw will be too high, or the charge rate too fast, for example. There's also a chance that the cells will be poorly manufactured and/or lacking the basic safety devices that even unprotected cells normally have. I don't recommend using them.