The ornamental design for a holder for a rack, as shown and described.
FIG. 1 is a perspective view of a holder for a rack, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a cross sectional perspective view thereof along line 7-7 in FIG. 2;
FIG. 8 is another cross sectional perspective view thereof along line 8-8 in FIG. 2; and,
FIG. 9 is a perspective view showing an embodiment of the claimed design, wherein the broken lines are shown for illustrative purposes only and form no part of the claimed design.