The ornamental design for a lining for a bag, as shown and described.
FIG. 1 is a plan view of a first embodiment of a lining for a bag showing my new design; and,
FIG. 2 is a plan view of a second embodiment of a lining for a bag showing my new design.
The broken lines are provided for illustrative purposes only and form no part of the claimed design. In addition, the portions of the lining for bag that are not shown in the drawings, if any, do not form any part of the claimed design.