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