I claim the ornamental design for an umbrella holder, as shown and described.
FIG. 1 is a back view of my invention;
FIG. 2 is side view thereof;
FIG. 3 is a front view thereof; and,
FIG. 4 is a side view thereof, showing the umbrella holder in use with an umbrella, the umbrella being shown in broken lines and forming no part of the claimed design.