The ornamental design for a futon warmer, as shown and described.
FIG. 1 is a front, top and right-side perspective view of a futon warmer showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a right-side view thereof;
FIG. 7 is a left-side view thereof; and,
FIG. 8 is a rear, top and left-side perspective view thereof.
The broken lines shown in the drawings depict portions of the futon warmer that form no part of the claimed design. The interior elements seen beyond the grill in FIG. 4 form no part of the claimed design.