The ornamental design for a decanter, as shown and described.
FIG. 1 is a perspective view of a decanter showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a plan view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a right side view thereof; and,
FIG. 8 is a left side view thereof.
Outer thick lines indicate outer shapes of the decanter and the head of the decanter, and inner thin lines indicate inner contour of the the decanter and the head of the decanter for illustration purpose.