The ornamental design for a base, as shown and described.
FIG. 1 is a top front perspective view of a base;
FIG. 2 is a top rear perspective view of the base shown in FIG. 1;
FIG. 3 is a front view of the base shown in FIG. 1;
FIG. 4 is a rear view of the base shown in FIG. 1;
FIG. 5 is a left side view of the base shown in FIG. 1;
FIG. 6 is a right side view of the base shown in FIG. 1;
FIG. 7 is a top view of the base shown in FIG. 1; and,
FIG. 8 is a bottom view of the base shown in FIG. 1.
The evenly broken lines are shown for the purpose of illustrating parts of the article that form no part of the claimed design. The dot-dash broken lines define bounds of the claimed design and form no part thereof.