The ornamental design for a dropfootpreventiondevice, as shown and described.
FIG. 1 is an upper right side perspective view of a drop foot prevention device according to the new design;
FIG. 2 is a top perspective view thereof; and,
FIG. 3 is an upper right side perspective view thereof, as worn by a person wearing a sandal.
The evenly dashed broken lines on the drop foot prevention device are for the purpose of illustrating stitching which forms no part of the claimed design. The unevenly dashed broken lines shown in FIG. 3 illustrate environmental subject matter and form no part of the claimed design.