The ornamental design for a computer, as shown and described.
FIG. 1 is a perspective view of a computer showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The single dot-dash lines shown immediately adjacent to the shaded areas in the figures define the boundaries of the claimed design, while all other broken lines are directed to environment and form no part of the claimed design.