Fiddle with the locking around channel disassociation

This still looks a bit sketchy to me (why isn't there locking in
port::destroy?) but this manages to get rid of a problem with channels
accessing their task after it's NULL.

src/rt/rust_chan.cpp

@@ -53,9 +53,8 @@ bool rust_chan::is_associated() {
 void rust_chan::disassociate() {
     A(kernel, is_associated(),
       "Channel must be associated with a port.");
-
-    if (port->is_proxy() == false) {
     scoped_lock with(port->referent()->lock);
+    if (port->is_proxy() == false) {
         KLOG(kernel, task,
              "disassociating chan: 0x%" PRIxPTR " from port: 0x%" PRIxPTR,
              this, port->referent());