The ornamental design for a hammer, as shown and described.
FIG. 1 is a perspective view of a hammer, showing my new design;
FIG. 2 is the other perspective view of the hammer in FIG. 1 thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The dashed lines depict environmental subject matter, are for illustration only and form no part of the claimed design. The dash-dot lines depict boundaries between the claimed part and non-claimed part and form no part of the claimed design.