The ornamental design for the sink drain plug, as shown and described.
FIG. 1 is a left side view of the present invention; and
FIG. 2 is a right side view of the present invention; and
FIG. 3 is a front view of the present invention; and
FIG. 4 is a rear view of the present invention; and
FIG. 5 is a top view of the present invention in its closed position; and
FIG. 6 is a top view of the present invention in its open position; and,
FIG. 7 is a perspective view of the present invention placed in an exemplary sink basin.
Wherein the dotted lines herein are illustrated for purpose of use and environment and form no part of the claimed invention and wherein the broken boundary line in FIG. 6 is shown for the purpose of illustrating the bounds of the claim and forms no part of the claimed design.