The ornamental design for a decanter, as shown and described.
FIG. 1 is a perspective view of a decanter showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of the bottom spike in the figures is included for the purpose of illustrating environment and forms no part of the claimed design.