The ornamental design for a golf bag, as shown and described.
FIG. 1 is a top, front, and right side perspective view of a golf bag showing my new design; and
FIG. 2 is a top, rear, and left side perspective view thereof, the broken line showings of golf clubs and covers are for illustrative purposes only and form no part of the claimed design.
Applicant disclaims the clubs and springs for urging the clubs upwardly.