The ornamental design for an extendablepillar, as shown and described.
FIG. 1 is a top, front perspective view of an extendable pillar showing my new ornamental design in an extended position with dashed broken lines representing unclaimed environmental subject matter that forms no part of the claimed design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a top, front perspective view of the extendable pillar in a compressed position with dashed broken lines representing unclaimed environmental subject matter that forms no part of the claimed design;