I claim the ornamental design for a leg warmer, as shown and described.
FIG. 1 is a perspective view of a leg warmer, showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective, environmental view thereof illustrating an example where the leg warmer is wrapped around a leg shown but disclaimed.
Broken lines are for illustrating unclaimed portions of the leg warmer and for illustrating an environment related to the claimed design; the broken lines form no part of the claimed design.