The ornamental design for a holster, as shown and described.
FIG. 1 is a top, rear, right side perspective view of a first embodiment of a holster according to our new design;
FIG. 2 is a bottom, front, right side perspective view thereof with additional environment;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The dot-dash broken lines represent boundaries of the claimed design. The broken lines shown in FIGS. 1-8 represent unclaimed portions of the holster that form no part of the claimed design. The additional broken lines shown in FIG. 2 illustrate environmental matter that forms no part of the claimed design.