The ornamental design for a liquid filter, as shown and described.
FIG. 1, is a side view showing my new design, the remaining sides being identical; and,
FIG. 2 is an end view, the opposite end being identical.
The broken lines adjacent to the shaded lines of FIG. 1 define the bounds of the claimed design and form no part thereof; the remaining broken lines on FIGS. 1 and 2 are for illustrative purposes only and form no part of the claimed design.