The ornamental design for an umbrellabase, as shown and described.
FIG. 1 is a back, top, and right side perspective view of an umbrella base embodying my new design.
FIG. 2 is a top view thereof.
FIG. 3 is a bottom view thereof.
FIG. 4 is a back view thereof.
FIG. 5 is a front view thereof.
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines are shown for the purpose of indicating environmental structure and/or portions of the umbrella base and form no part of the claimed design.