The ornamental design for "CD holder", as shown and described.
FIG. 1 is a front plain view of a "CD holder" showing my new design, the back plain view being identical thereto;
FIG. 2 is a left plain view of a "CD holder" showing my new design, the right plain view being identical thereto; and,
FIG. 3 is a front perspective view thereof, with a broken line showing of CD cases being for illustrative purposes only and forming no part of the claimed design.