The ornamental design for an interior poststiffener, as shown and described.
FIG. 1 is a front right perspective view of an interior post stiffener;
FIG. 2 is a top plan view thereof, a bottom plan view being a mirror image;
FIG. 3 is a front side plan view thereof, a rear side, right side, and left side plan view being a mirror image; and,
FIG. 4 is a front right perspective view of an interior post stiffener shown within an environment of a post. The broken lines showing a post is for the purpose of illustrating the environmental structure and forms no part of the claimed design.