I claim the ornamental design for a protective guard for a watch, as shown and described.
FIG. 1 is a perspective view of the protective guard for a watch according to my design showing the environment in which the protective guard is used;
FIG. 2 is a perspective view of the protective guard for a watch according to my design;
FIG. 3 is a front view of the protective guard for a watch according to my design;
FIG. 4 is a back view of the protective guard for a watch according to my design;
FIG. 5 is a right side view of the protective guard for a watch according to my design;
FIG. 6 is a left side view of the protective guard for a watch according to my design;
FIG. 7 is a top plan view of the protective guard for a watch according to my design; and,
FIG. 8 is a bottom plan view of the protective guard for a watch according to my design.
The broken line showing in FIG. 1 is for illustrative purposes only to show the environment in which the protective guard is used, and forms no part of the claimed design.