The ornamental design for a crutch, as shown and described.
FIG. 1 is a top rear perspective view of a crutch.
FIG. 2 is a back elevation view of the crutch.
FIG. 3 is a front elevation view of the crutch.
FIG. 4 is a right side elevation view of the crutch.
FIG. 5 is a left side elevation view of the crutch.
FIG. 6 is a top plan view of the crutch; and,
FIG. 7 is a bottom plan view of the crutch.
The broken lines showing portions of the crutch are included for the purposes of illustrating portions of the article and form no part of the claimed design.