The ornamental design for an emergency flag, as shown and described.
FIG. 1 is a front elevation view of an emergency flag showing my new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is an isometric view thereof.
The broken lines in FIG. 7 of the drawings depict portions of a vehicle as environmental subiect matter only and form no part of the claimed design.