The ornamental design for a computer, as shown and described.
FIG. 1 is a side perspective view of a computer;
FIG. 2 is another side perspective view of the computer;
FIG. 3 is another side perspective view of the computer; and,
FIG. 4 is a rear perspective view of the computer.
The broken lines showing of various regions and the unshaded areas therein are for the purpose of illustrating the article and form no part of the claimed design.