The ornamental design for a mirror, as shown and described.
FIG. 1 is a top view of a mirror;
FIG. 2 is a front end view thereof;
FIG. 3 is a rear end view thereof;
FIG. 4 is a side view thereof; and,
FIG. 5 is an illustrative view of a mirror showing the design in a position of use, affixed to the underside of a toilet seat cover. The broken line showing of the toilet seat, cover, and hinges in FIG. 5 is for illustrative purposes only and forms no part of the claimed design.