The ornamental design for a knee warmer, as shown and described.
FIG. 1 is a left front perspective view of my present invention.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a left side elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a top plan view thereof.
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a front left perspective view thereof in an use position.
No claim is made to portions that are not shown. The dash-dash broken lines showing in the figures are included for the purpose of illustrating environmental structure of the knee warmer and form no part of the claimed design. The dash-dot-dot-dash lines on the interior of the knee warmer indicate boundary lines and no claim is made therein. The surfaces are lined for contour.