The ornamental design for a doorstop, as shown and described.
FIG. 1 is an upper perspective view of a door stop showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a lower perspective view thereof.
The features presented in broken lines, including the magnet on the front side of the door stop, represent unclaimed portions of the door stop and form no part of the claimed design.