The ornamental design of a collar for a bib, as shown and described.
FIG. 1 is a right perspective view of the collar attached to a bib;
FIG. 2 is a front view of the collar attached to a bib;
FIG. 3 is a back view of the collar attached to a bib;
FIG. 4 is a left side view of the collar attached to a bib;
FIG. 5 is a right side view of the collar attached to a bib;
FIG. 6 is a top view of the collar attached to a bib; and,
FIG. 7 is a bottom view of the collar attached to a bib.
The broken lines in FIGS. 1, 2, 5 and 6, on the surface of the collar, represent stitching. The remaining broken lines in FIGS. 1-7 showing a bib represent the portions of the design, which form no part of the claim.