Minor adb style fixes.

Change-Id: Ic4b45828dfdf88ef6c0fc468f66df3e62d2fbe51
diff --git a/adb/sockets.cpp b/adb/sockets.cpp
index b2555d0..4ed1c45 100644
--- a/adb/sockets.cpp
+++ b/adb/sockets.cpp
@@ -410,7 +410,7 @@
 #endif
     int fd = service_to_fd(name, transport);
     if (fd < 0) {
-        return 0;
+        return nullptr;
     }
 
     asocket* s = create_local_socket(fd);