The ornamental design for a muffler, substantially as shown and described.
FIG. 1 is a perspective view of a muffler showing an embodiment of the new design;
FIG. 2 is a first side view of an embodiment of the muffler;
FIG. 3 is a second side view of an embodiment of the muffler;
FIG. 4 is a front view of an embodiment of the muffler;
FIG. 5 is a top view of an embodiment of the muffler; and,
FIG. 6 is a bottom view of an embodiment of the muffler.
The broken line showing of aspects of the muffler in FIG. 6 is included for the purpose of illustrating environment and forms no part of the claimed design.