The ornamental design for an adjustable studbracket, as shown and described.
FIG. 1 is a perspective view of an adjustable stud bracket of my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawings define the bounds of the claimed design and form no part thereof. The adjustable stud bracket is shown with symbolic breaks in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.