The ornamental design for a mask, as shown and described.
FIG. 1 is a perspective view of an ornamental design of one embodiment of a mask showing our new design.
FIG. 2 is a front orthographic view thereof.
FIG. 3 is a rear orthographic view thereof.
FIG. 4 is a right side orthographic view thereof.
FIG. 5 is a left side orthographic view thereof.
FIG. 6 is a top orthographic view thereof; and,
FIG. 7 is a bottom orthographic view thereof.
The broken lines shown in FIGS. 1 and 3-7 are included for the purpose of illustrating environmental elements only and form no part of the claimed design.