The ornamental design for a dehumidifier, as shown and described.
FIG. 1 is a first perspective view of a dehumidifier showing my new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is an enlarged view of the selected portion 9 in FIG. 1.
The dashed broken lines in the drawings depict portions of the dehumidifier that form no part of the claimed design. The dash dot dash lines showing the enlargements in FIGS. 1 and 9 farm no part of the claimed design.