I claim the ornamental design for a neck warmer, as shown and described.
FIG. 1 is a perspective front view of a neck warmer being worn in the down configuration;
FIG. 2 is a perspective front view of the invention being worn in the up configuration;
FIG. 3 is a back view of the invention in a clasped-closed configuration; and,
FIG. 4 is a back view of the invention in a laid open configuration.
The large dashed lines shown in the drawings depict the wearer of the neck warmer and form no part of the claimed design. The small dashed lines shown in the drawings depict stitching and form no part of the claimed design.