"adb tcpip" should require a numeric argument.

Defaulting to port 0 just breaks stuff.

Bug: http://b/22636927
Change-Id: I6239900e0828e71b31171d0184c24824957c99c8
diff --git a/services.cpp b/services.cpp
index b9c532a..2e3ad98 100644
--- a/services.cpp
+++ b/services.cpp
@@ -486,7 +486,7 @@
     } else if(!strncmp(name, "tcpip:", 6)) {
         int port;
         if (sscanf(name + 6, "%d", &port) != 1) {
-            port = 0;
+            return -1;
         }
         ret = create_service_thread(restart_tcp_service, (void *) (uintptr_t) port);
     } else if(!strncmp(name, "usb:", 4)) {