Drop the reference to best when freeing the foreign masters.

This fixes the following issue reported by valgrind, which occurs
after a port disable/initialize subsequent to having entered slave
mode.

==10651== Invalid read of size 4
==10651==    at 0x804E6E2: fc_clear (port.c:175)
==10651==    by 0x805132F: port_event (port.c:1352)
==10651==    by 0x804B383: clock_poll (clock.c:597)
==10651==    by 0x80498AE: main (ptp4l.c:278)
==10651==  Address 0x41cba60 is 16 bytes inside a block of size 60 free'd
==10651==    at 0x4023B6A: free (vg_replace_malloc.c:366)
==10651==    by 0x804EB09: free_foreign_masters (port.c:287)
==10651==    by 0x804FB14: port_disable (port.c:722)
==10651==    by 0x8051228: port_dispatch (port.c:1298)
==10651==    by 0x804B3C6: clock_poll (clock.c:602)
==10651==    by 0x80498AE: main (ptp4l.c:278)

Signed-off-by: Richard Cochran <[email protected]>

port.c

@@ -719,6 +719,7 @@ static void port_disable(struct port *p)
 		p->peer_delay_fup = NULL;
 	}
 
+	p->best = NULL;
 	free_foreign_masters(p);
 	clock_remove_fda(p->clock, p, p->fda);
 	transport_close(p->trp, &p->fda);