The ornamental design for a massagestone warmer, as shown and described.
FIG. 1 is a perspective view of a massage stone warmer showing my new design where the massage stone warmer is in an open state;
FIG. 2 is a perspective view of the massage stone warmer where the massage stone warmer is in a close state;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken line shown in the drawings is included for the purpose of illustrating portions of the massage stone warmer and forms no part of the claimed design.