pimd: fix UAF/heap corruption in BSM code

pimd: fix UAF/heap corruption in BSM code

This `XFREE()` call is in plainly in the wrong spot.  `rp_all` (the
224.0.0.0/4 entry) isn't supposed to be free'd ever, and the
conditional above makes quite clear that it remains in use.

It may be possible to exploit this as a heap corruption bug, maybe even
as RCE.  I haven't tried; I randomly noticed this while working on the
BSM code.  Luckily this code is only run by the CLI for the clear
command, so the surface is very small.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>