adb: avoid crash when authenticating without vendor keys.

Bug: http://b/158232136
Test: manual
Change-Id: Ic8211335494a6acb703af5b82f52856223f538bb
diff --git a/adb/transport.cpp b/adb/transport.cpp
index b6b6984..c33d5af 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -1533,8 +1533,7 @@
         keys_.pop_front();
     }
 
-    std::shared_ptr<RSA> result = keys_[0];
-    return result;
+    return Key();
 }
 
 void atransport::ResetKeys() {