The ornamental design for a finger warmer, as shown and described.
FIG. 1 is a front perspective view of a finger warmer, with sides folded together, showing our new design;
FIG. 2 is a front view thereof (with sides folded together);
FIG. 3 is a top view thereof (with sides folded together);
FIG. 4 is a bottom view thereof (with sides folded together);
FIG. 5 is a side view thereof (with sides folded together);
FIG. 6 is a rear view thereof (with sides folded together);
FIG. 7 is a top view thereof (with sides extended); and,
FIG. 8 is a bottom view thereof (with sides extended).
The broken lines depicting a dog's head as well as the broken lines shown on the inner and outer surfaces of the mitt body illustrate those portions of the article in which the design is embodied that form no part of the claim.