powermand: fix assertion failure on teardown

Problem: an assertion failure is seen when powermand tears
down with a tcp-connected device:

powermand: device_tcp.c:266: tcp_disconnect:
  Assertion `dev->magic == DEV_MAGIC' failed.

This is because dev_destroy sets dev->magic = 0 before it calls
dev->disconnect.  When dev->disconnect points to tcp_disconnect()
the assertion triggers.

Fixes #106

src/powerman/device.c

@@ -1306,7 +1306,6 @@ void dev_destroy(Device * dev)
  int i;
 
  assert(dev->magic == DEV_MAGIC);
- dev->magic = 0;
 
  if (dev->connect_state == DEV_CONNECTED)
  dev->disconnect(dev);
@@ -1327,6 +1326,7 @@ void dev_destroy(Device * dev)
  cbuf_destroy(dev->to);
  cbuf_destroy(dev->from);
  xregex_match_destroy(dev->xmatch);
+ dev->magic = 0;
  xfree(dev);
 }