I claim the ornamental design for a pre-assembled wall frame, as shown and described.
FIG. 1 is a front top perspective view of the claimed design;
FIG. 2 is a front view of the claimed design of FIG. 1;
FIG. 3 is a left side view of the claimed design of FIG. 1; and,
FIG. 4 is a right side view of the claimed design of FIG. 1.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines depict environment that forms no part of the claimed design.