The ornamental design for a mat, as shown and described.
FIG. 1 is a front perspective view of a mat showing my design;
FIG. 2 is a right side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a front view thereof;
FIG. 7 is a rear view thereof; and,
FIG. 8 is a rear perspective view thereof.
The broken lines shown in the figures are for illustrative purposes to depict unclaimed features of the mat only and form no part of the claimed design.