The ornamental design for a flask, as shown and described.
FIG. 1 is a front elevated perspective view of a flask showing our new design (wherein the dashed lines represent an optional central window having a doughnut-shaped frame).
FIG. 2 is a front view of the flask of FIG. 1.
FIG. 3 is a rear view of the flask of FIG. 1.
FIG. 4 is a left side view of the flask of FIG. 1.
FIG. 5 is a right side view of the flask of FIG. 1.
FIG. 6 is a top view of the flask of FIG. 1; and,
FIG. 7 is a bottom view of the flask of FIG. 1.
An optional window having a doughnut-shaped frame is shown in broken lines for the purpose of illustrating environmental structure only and forms no part of the claimed design.