The ornamental design for a holster, as shown and described.
FIG. 1 is a front elevation view of the invention;
FIG. 2 is a rear elevation view of the invention;
FIG. 3 is a top plan view of the invention;
FIG. 4 is a bottom plan view of the invention;
FIG. 5 is a left side view of the invention;
FIG. 6 is a right side view of the invention;
FIG. 7 is a perspective view of the invention; and,
FIG. 8 is a perspective view of the invention in use.
In the drawings, the broken lines illustrate features that form no part of the claimed design and unclaimed environmental subject matter that form no part of the claim.