The ornamental design for a heart-shaped knife stand, as shown and described.
FIG. 1 is a front elevational view of a heart-shaped knife stand showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a front elevational view thereof with knives inserted in the knife stand. The broken lines shown on the drawings form no part of the claimed design.