The ornamental design for a holster, as shown and described.
FIG. 1 is a front perspective view of a holster according to my new design.
FIG. 2 is a front view of the holster of FIG. 1.
FIG. 3 is a rear view of the holster of FIG. 1.
FIG. 4 is a right side view of the holster of FIG. 1.
FIG. 5 is a left side view of the holster of FIG. 1.
FIG. 6 is a top view of the holster of FIG. 1; and,
FIG. 7 is a bottom view of the holster of FIG. 1.
The features shown in smaller broken lines represent environmental matter and do not form part of the claimed design. The features shown in larger broken lines are stitch lines that do form part of the claimed design.