Show "(reverse)" in "adb reverse --list".

Bug: http://b/17788586
Change-Id: I250a1459295ed38e8d962da8bea2a8d2f83c7ef0
diff --git a/adb/adb_listeners.cpp b/adb/adb_listeners.cpp
index 641b537..a15c513 100644
--- a/adb/adb_listeners.cpp
+++ b/adb/adb_listeners.cpp
@@ -142,8 +142,10 @@
             continue;
         }
         //  <device-serial> " " <local-name> " " <remote-name> "\n"
+        // Entries from "adb reverse" have no serial.
         android::base::StringAppendF(&result, "%s %s %s\n",
-                                     l->transport->serial, l->local_name, l->connect_to);
+                                     l->transport->serial ? l->transport->serial : "(reverse)",
+                                     l->local_name, l->connect_to);
     }
     return result;
 }