The ornamental design for an air hammer, as shown and described.
FIG. 1 is a perspective view of an air hammer showing my new design;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof when in use.
The broken circles shown in FIG. 4 and the broken lines shown in FIG. 8 are provided for the purpose of illustrating environmental structure and forms no part of the claimed design.