The ornamental design for a hammer, as shown and description.
FIG. 1 is a isometric front view of the hammer showing one face of the hammer.
FIG. 2 is a isometric back view of the hammer showing another face of the hammer.
FIG. 3 is a side view of the hammer.
FIG. 4 is the other side view of the hammer.
FIG. 5 is the front view of the hammer showing one face of the hammer.
FIG. 6 is a back view of the hammer showing another face of the hammer.
FIG. 7 is a top view of the hammer.
FIG. 8 is bottom view of the hammer; and,
FIG. 9 is a isometric front view of the hammer showing a nail attached to the magnetic face of the hammer.
The broken line showing of the nail in FIG. 9 is provided for environmental purposes and forms no part of the claimed design. The handle and the front face are shown in broken line form because they depict unclaimed portions of the hammer.