Correctly handle link detach and session end.

It was possible for `Session.mux` to return before an associated link.
Due to an oversight during refactoring the detach logic did not account
for this and the link could deadlock attempting to send the response
Detach.

client.go

@@ -1008,8 +1008,10 @@ func (l *link) detach() {
 	}
 	select {
 	case l.session.tx <- fr:
-	case <-l.session.conn.done:
-		l.err = l.session.conn.getErr()
+	case <-l.session.done:
+		if l.err == nil {
+			l.err = l.session.err
+		}
 		return
 	}
 	l.detachSent = true
@@ -1018,8 +1020,10 @@ func (l *link) detach() {
 	if l.detachReceived {
 		select {
 		case l.session.deallocateHandle <- l:
-		case <-l.session.conn.done:
-			l.err = l.session.conn.getErr()
+		case <-l.session.done:
+			if l.err == nil {
+				l.err = l.session.err
+			}
 		}
 		return
 	}
@@ -1037,16 +1041,22 @@ outer:
 			}
 
 		// connection has ended
-		case <-l.session.conn.done:
-			l.err = l.session.conn.getErr()
+		case <-l.session.done:
+			if l.err == nil {
+				l.err = l.session.err
+			}
+			return
 		}
 	}
 
 	// deallocate handle
 	select {
 	case l.session.deallocateHandle <- l:
-	case <-l.session.conn.done:
-		l.err = l.session.conn.getErr()
+	case <-l.session.done:
+		if l.err == nil {
+			l.err = l.session.err
+		}
+		return
 	}
 }