The ornamental design for a frozen confectionary, as shown and described.
 FIG. 1 is a perspective view of a frozen confectionary in accordance with the present invention;
 FIG. 2 is a rear elevation view thereof;
 FIG. 3 is a front elevation view thereof;
 FIG. 4 is a top plan view thereof;
 FIG. 5 is a bottom plan view thereof;
 FIG. 6 is a right side elevation view thereof; and,
 FIG. 7 is a left side elevation view thereof.