The ornamental design for a casing cornerpiece, as shown and described.
FIG. 1 is an upper perspective view of the casing corner piece of the present design;
FIG. 2 is a lower perspective of the casing corner piece of FIG. 1;
FIG. 3 is a front view of the casing corner piece of FIG. 1;
FIG. 4 is a right side view of the casing corner piece of FIG. 1;
FIG. 5 is a back view of the casing corner piece of FIG. 1;
FIG. 6 is a left side view of the casing corner piece of FIG. 1;
FIG. 7 is a top view of the casing corner piece of FIG. 1;
FIG. 8 is a bottom view of the casing corner piece of FIG. 1; and,
FIG. 9 is a front perspective view of the casing corner piece of FIG. 1, shown in combination with horizontal and vertical casing members illustrated in broken line, the portions shown in broken line helping to illustrate environment and forming no part of the claimed design.