I claim the ornamental design for mirror, as shown.
FIG. 1 is a perspective view of a mirror according to an embodiment of the invention.
FIG. 2 is a front elevational view of a mirror according to an embodiment of the invention.
FIG. 3 is a top plan view of a mirror according to an embodiment of the invention; and,
FIG. 4 is a right side elevational view of a mirror according to an embodiment of the invention. The left side view is a mirror image of this right side.
The dashed lines are not part of the claimed invention. The back view of the mirror has no further claimed features and is not illustrated.