The ornamental design for a guardrail for a bed, as shown and described.
FIG. 1 is a perspective view of a guard rail for a bed showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a left side elevation view thereof;
FIG. 8 is a right side elevation view thereof; and,
FIG. 9 shows an environment in which the guard rail for a bed is used.
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.