I claim the ornamental design for a muffler, as shown and described herein.
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FIG. 1 is a perspective view of a muffler according to the new design.
FIG. 2 is a side view of the muffler according to the new design.
FIG. 3 is a top view of the muffler according to the new design; and,
FIG. 4 is a bottom view of the muffler according to the new design.