The ornamental design for a flask, as shown and described.
FIG. 1 is a bottom view of a flask showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front elevational view thereof; and,
FIG. 4 is a left side perspective view thereof.
The broken lines in the drawings depict environmental subject matter only and form no part of the claimed design.
All sides not shown form no part of the claimed design.