The ornamental design for a nebulizer, as shown and described.
FIG. 1 is a perspective view showing my design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a rear elevational view thereof.
The broken lines shown in FIGS. 1, 2 and 6 which are immediately adjacent to claimed surfaces, represent the bounds of the claimed design, which form no part thereof.