I claim the ornamental design for a holder for a portable computer, as shown and described.
FIG. 1 is a front perspective view of a holder for a portable computer of the present invention;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a first side view thereof;
FIG. 5 is a second side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view thereof shown in an environmental use.
The broken line showing of portions of a holder for a portable computer and a portable computer is included for the purpose of illustrating environment and forms no part of the claimed design. The uneven broken lines in the drawings define the limits of the claimed design.