We claim the ornamental design for a hammer, as shown and described.
FIG. 1 is a perspective view of a hammer embodying our new design.
FIG. 2 is a right side view of the hammer of FIG. 1.
FIG. 3 is a left side view of the hammer of FIG. 1.
FIG. 4 is a front view of the hammer of FIG. 1.
FIG. 5 is a rear view of the hammer of FIG. 1.
FIG. 6 is a top view of the hammer of FIG. 1; and,
FIG. 7 is a bottom view of the hammer of FIG. 1.
The broken lines and the dot-dash boundary depict portions of the hammer in which the design is embodied that are not considered part of the claimed design.