The ornamental design for a strapendretainer, as shown and described.
FIG. 1 is a front-right side perspective view of a strap end retainer showing my new design;
FIG. 2 is a front-right side perspective view thereof showing the retainer deployed around a strap end with no claim to the portions shown in dashed lines;
FIG. 3 is a front side elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a right side elevational view thereof; and,