The ornamental design for a loftbed, as shown and described.
FIG. 1 is a perspective view of a loft bed showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line elements in the drawings are shown solely to illustrate environments in which my new design is used, and form no part of the claimed design.