We claim the ornamental design for a fence post brace, as shown and described.
FIG. 1 is a top perspective view of a fence post brace.
FIG. 2 is a side elevation view thereof showing environmental structure; and,
FIG. 3 is an enlarged view of the fence post brace.
Broken lines are included for the purpose of illustrating portions of the combination fence brace article and/or environmental structure that form no part of the claimed design.