update_engine: Fix issues reported by clang-tidy

Fix uninitialized variables and some other minor issues reported by
clang-tidy.

BUG=chromium:982837
TEST=cros_workon_make update_engine --test

Change-Id: I305dedb058c9b0787ba2f68feff42afe6810a276
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2122683
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Andrew Lassalle <andrewlassalle@chromium.org>
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
diff --git a/update_engine_client.cc b/update_engine_client.cc
index eb71841..a721f7a 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -202,7 +202,7 @@
 
 void UpdateWaitHandler::HandleStatusUpdate(const UpdateEngineStatus& status) {
   if (exit_on_error_ && status.status == UpdateStatus::IDLE) {
-    int last_attempt_error;
+    int last_attempt_error = static_cast<int>(ErrorCode::kSuccess);
     ErrorCode code = ErrorCode::kSuccess;
     if (client_ && client_->GetLastAttemptError(&last_attempt_error))
       code = static_cast<ErrorCode>(last_attempt_error);