Check returned SafeBrowsingPrivateEventRouter for null

SafeBrowsingPrivateEventRouterFactory::GetForProfile() can return
nullptr, calling methods on nullptr is undefined behavior and causes
some tests in unit_tests to crash under UBSan VPtr.

Fixes
$ ./out/Default/unit_tests --gtest_filter='*ChromeDownloadManagerDelegateTestWithSafeBrowsing*'
under UBSan VPtr

Bug: 1137496
Change-Id: I461f5d1235da4bc98bce22a1d7a4183ed11c4f25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568667Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/master@{#833160}

chrome/browser/download/chrome_download_manager_delegate.cc

@@ -358,13 +358,16 @@ void MaybeReportDangerousDownloadBlocked(
     return;
   }
 
-  std::string raw_digest_sha256 = download->GetHash();
-  extensions::SafeBrowsingPrivateEventRouterFactory::GetForProfile(profile)
-      ->OnDangerousDownloadEvent(
-          download->GetURL(), download_path,
-          base::HexEncode(raw_digest_sha256.data(), raw_digest_sha256.size()),
-          danger_type, download->GetMimeType(), download->GetTotalBytes(),
-          safe_browsing::EventResult::BLOCKED);
+  auto* router =
+      extensions::SafeBrowsingPrivateEventRouterFactory::GetForProfile(profile);
+  if (router) {
+    std::string raw_digest_sha256 = download->GetHash();
+    router->OnDangerousDownloadEvent(
+        download->GetURL(), download_path,
+        base::HexEncode(raw_digest_sha256.data(), raw_digest_sha256.size()),
+        danger_type, download->GetMimeType(), download->GetTotalBytes(),
+        safe_browsing::EventResult::BLOCKED);
+  }
 #endif
 }