The ornamental design for a bagelslicer, as shown.
FIG. 1 is a top front perspective view of a bagel slicer of my new design;
FIG. 2 is a bottom front perspective view of FIG. 1;
FIG. 3 is a front view of FIG. 1;
FIG. 4 is a side view of FIG. 1;
FIG. 5 is a top view of FIG. 1; and,
FIG. 6 is a bottom view of FIG. 1. The broken lines in FIG. 6 are included for the purpose of illustrating a portion of the bagel slicer that forms no part of the claimed design.