FIG. 1 is a front perspective view of a bag of the claimed design; FIG. 2 is a front view of the bag shown in FIG. 1; FIG. 3 is a rear view of the bag shown in FIG. 1; FIG. 4 is a right side view of the bag shown in FIG. 1; FIG. 5 is a left side view of the bag shown in FIG. 1; FIG. 6 is a top view of the bag shown in FIG. 1; and, FIG. 7 is a bottom view of the bag shown in FIG. 1.