The ornamental design for a decanter, as shown and described.
FIG. 1 is a top-front-left perspective view of a decanter showing my new design;
FIG. 2 is a bottom-rear-right perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a right-side elevational view thereof;
FIG. 7 is a front elevational view thereof; and,
FIG. 8 is a rear elevational view thereof.
The dashed line showing a cap in FIGS. 1, 2, 3, 5, 6, 7, and 8 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.