I claim the ornamental design for a strainer for a bathtub drain, as shown and described.
FIG. 1 is a top, perspective view of a strainer for a bathtub drain, of the claimed design;
FIG. 2 is a bottom, perspective view of the strainer for a bathtub drain of FIG. 1;
FIG. 3 is a top view of the strainer for a bathtub drain of FIG. 1;
FIG. 4 is a bottom view of the strainer for a bathtub drain of FIG. 1;
FIG. 5 is a side view of the strainer for a bathtub drain of FIG. 1; and,
FIG. 6 is another side view of the strainer for a bathtub drain of FIG. 1 (with the bathtub strainer being rotated by 90-degrees relative to its position FIG. 5).