I claim, the ornamental design for an earplug stem, as shown and described.
FIG. 1 is a perspective view of an earplug stem, showing my new design,
FIG. 2 is a first side elevation view thereof, the opposite side being a mirror image thereof;
FIG. 3 is a second side elevation view thereof, the opposite side being a mirror image thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken line showing forms no part of the claimed design. The surface shading is provided only to indicate contour. It is not indicative of texture, gloss, etc.