procstat: CID 1593951: Resource leak

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

procstat: CID 1593951: Resource leak

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.

Reviewed-bys: vangyzen
Differential Revision: https://reviews.freebsd.org/D55422

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