FIG. 1 illustrates a left side view of the claimed design for a hot dog slicer; FIG. 2 illustrates a front view thereof; FIG. 3 illustrates a rear view thereof; FIG. 4 illustrates a right-side view thereof; FIG. 5 illustrates a top view thereof; FIG. 6 illustrates a bottom view thereof; FIG. 7 illustrates a cross-sectional view thereof, taken along line 7-7 of FIG. 5; FIG. 8 illustrates a perspective view thereof; and, FIG. 9 illustrates an exploded view thereof.