The ornamental design for a boot warmer, as shown and described.
FIG. 1 shows a top plan view of a boot warmer showing my new design,;
FIG. 2 shows a right side view thereof, the view from the left side being a mirror image thereto;
FIG. 3 shows a bottom view thereof;
FIG. 4 shows a rear end view thereof, taken from the left side as depicted in FIGS. 1-3; and,
FIG. 5 shows a front end view thereof, taken from the right side as depicted in FIGS. 1-3.
The boot warmer of the invention is used by heating it in a microwave oven or otherwise and placing it in a ski boot or the like to warm it prior to being worn.