The ornamental design for a flask, as shown and described.
FIG. 1 is a top plan view of a flask showing my new design;
FIG. 2 is a front, rear, and lateral elevational view thereof;
FIG. 3 is a perspective view thereof; and,
FIG. 4 is a bottom plan view thereof.
The broken line showing is included for the purpose of environment only and forms no part of the claimed design.