The ornamental design for a dehumidifier, as shown and described.
FIG. 1 is a front perspective view of a dehumidifier according to my new design;
FIG. 2 is a front elevational view of the dehumidifier shown in FIG. 1, a rear elevational view not being shown as it is devoid of ornamentation;
FIG. 3 is a right side elevational view of the dehumidifier shown in FIG. 1, the left side elevational view being a mirror image of the right side elevational view; and,
FIG. 4 is a top plan view of the dehumidifier shown in FIG. 1, a bottom plan view not being shown as it is devoid of ornamentation.
The broken lines in FIGS. 1-3 are for illustrative purposes only and form no part of the claimed design.