The ornamental design for an outside flash, as shown and described.
FIG. 1 is a first perspective view of an outside flash showing my design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The dashed or broken lines shown in the drawings depict portions of the outside flash in which the design is embodied that form no part of the claimed design.