The ornamental design for a shelter, as shown and described.
FIG. 1 is a front elevational view of a shelter showing our new design, the opposite side being a mirror image thereof;
FIG. 2 is a side elevational view thereof;
FIG. 3 is a side elevational view of a side opposite that of FIG. 2;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a front perspective view thereof.
The broken line showing of details in FIGS. 1–6 is included for the purpose of showing environmental portions of the shelter and forms no part of the claimed design.