The ornamental design for bunk bed stair, as shown and described.
FIG. 1 is a left, front perspective view of the bunk bed stair showing my new design;
FIG. 2 is a left, rear perspective view thereof;
FIG. 3 is a left, rear perspective view thereof;
FIG. 4 is a left view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a perspective view of the bunk bed stair showing my new design; the broken lines shown in FIG. 7 are for the purpose of illustrating the environment in which the bunk bed stair are associated and form no part of the claimed design.