We claim the ornamental design for a flask, as shown and described.
FIG. 1 is a perspective view of a flask showing our new design;
FIG. 2 is an exploded view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The even dash-dash lines in FIGS. 1-7 illustrate portions of the flask that form no part of the claimed design. The even dash-dash lines in FIG. 8 represent bounds of the claimed design which, and inside which, form no part of the claimed design.