We claim the ornamental design for a holster, as shown and described.
FIG. 1 is a front perspective view of a holster;
FIG. 2 is a rear perspective view of the holster;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right elevational view thereof;
FIG. 6 is a left elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines shown in the drawings of the holster are for the purpose of illustrating portions of the article and form no part of the claimed design.