The ornamental design for a triggerguard for a firearm, as shown and described.
FIG. 1 is a bottom-side perspective view of a trigger guard for a firearm in accordance with the new design;
FIG. 2 is a left side view of the trigger guard for a firearm of FIG. 1;
FIG. 3 is a right side view of the trigger guard for a firearm of FIG. 1;
FIG. 4 is a front view of the trigger guard for a firearm of FIG. 1; and,
FIG. 5 is a bottom view of the trigger guard for a firearm of FIG. 1.
The even length broken lines in the figures are directed to portions of a firearm that form no part of the claimed design. The dot-dashed lines show unclaimed boundaries of the trigger guard for a firearm and form no part of the claimed design.