The ornamental design for a handle for a knife, as shown and described.
FIG. 1 is a front elevational view of a handle for a knife, showing my new design;
FIG. 2 is a rear elevational view of my design;
FIG. 3 is a right side elevational view of my design;
FIG. 4 is a left side elevational view of my design;
FIG. 5 is a top plan view of my design;
FIG. 6 is a bottom plan view of my design; and,
FIG. 7 is a top right side perspective view of my design, in which the broken lines depict an exemplary blade element as mounted thereto as environmental structure for illustrative purposes only and form no part of the claimed design.