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