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 a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dash-dot-dash broken lines on the bottom plan view of FIG. 7 represent bounds of the claimed design, inside which forms no part of the claimed design. The other dash-dash broken lines on the bottom plan view of FIG. 7 are directed to unclaimed features and form no part of the claimed design.