procstat: CID 1593951: Resource leak

[dab]

Summary

A trivial resource leak fix; free the allocated memory before return.

Test Plan

Code inspection, run command.

I built a simple program that waits for a signal on a kqueue, then ran that. The standard procstat displays:

fbsd-dev% Waiting for SIGTERM...
procstat -a kqueue
    PID       KQFD   FILTER      IDENT      FLAGS     FFLAGS       DATA      UDATA     STATUS
  84352          3   SIGNAL         15          C          -          0        0x0          -

The revised procstat displays:

fbsd-dev% sudo LD_LIBRARY_PATH=/usr/obj/usr/home/dab/git/freebsd/src/arm64.aarch64/lib/libutil /usr/obj/usr/home/dab/git/freebsd/src/arm64.aarch64/usr.bin/procstat/procstat -a kqueue
    PID       KQFD   FILTER      IDENT      FLAGS     FFLAGS       DATA      UDATA     STATUS
  84352          3   SIGNAL         15          C          -          0        0x0          -
fbsd-dev%

As expected, the two displays are identical. This doesn't prove that the leak is gone, but it does prove that the revised command still operates correctly. I think it can clearly be seen from inspection of the change that the leak has been remedied.