The ornamental design for a convertiblecrib, as shown and described.
FIG. 1 is a perspective view of a convertible crib, showing my new design, the broken lines showing the vertical slats and horizontal bottom rail of the headboard are for environmental purposes only and form no part of the claimed design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation view thereof; and,
FIG. 4 is a left side elevation view thereof; the right side elevation view is a mirror image of the left side elevation view and is therefore not illustrated.
The back of the headboard and underside of the convertible crib are not illustrated as not being part of the claimed design.