I claim the ornamental design for a knee pad, as shown and described.
FIG. 1 is a perspective view of a knee pad, showing my new design;
FIG. 2 is a top plan view of the invention;
FIG. 3 is a bottom plan view of the invention;
FIG. 4 is a front elevational view of the invention;
FIG. 5 is a rear elevational view of the invention;
FIG. 6 is a left-side elevational view of the invention; and,
FIG. 7 is a right-side elevational view of the invention.
The broken lines showing a crawling baby in FIG. 1 are included for the purpose of illustrating a knee pad in use, and forms no part of the claimed design.