The ornamental design for a core for a pillow, as shown and described.
1. Core for a pillow
1.1—Front perspective view in a flattened position
1.2—Front perspective view in a curved position
1.3—Another front perspective view in a flattened position
1.4—Front
1.5—Back
1.6—Top
1.7—Bottom
1.8—Left
1.9—Right
The features shown in dotted lines depict environmental subject matter only and form no part of the claimed design; the lines having alternating long and short segments define bounds of the claimed design and form no part thereof; thin lines indicated on surfaces of this article in each reproduction are for identifying shapes of three-dimensional surfaces.