I claim, the ornamental design for a protective casing, as shown and described.
FIG. 1 is a front perspective view of a protective casing, showing my new design, and including a lower portion shown herein and throughout in broken lines for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof taken from the right of FIG. 2;
FIG. 5 is a side view thereof taken from the left of FIG. 2;