FIG. 1 is a front perspective view of a curtain fastener showing my new design; FIG. 2 is a top view thereof; FIG. 3 is a front view thereof; FIG. 4 is a right-side view thereof; FIG. 5 is a left-side view thereof; FIG. 6 is a rear view thereof; and, FIG. 7 is a bottom view thereof. The broken lines are for illustrative purposes only and form no part of the claimed design.