Add return functions for preventing crash

Previous codes do not return directly even if
plugin_instance and load_manager are null.
It can occur crash whenever accessing each variables.
This patch add return codes for avoiding crash
if variable is null.

BUG=NONE

Change-Id: Ia5c59a9473124d6ad1ff76ee909d9396af35e96c
Reviewed-on: https://chromium-review.googlesource.com/586488Reviewed-by: Raymes Khoury <raymes@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491067}

components/nacl/renderer/ppb_nacl_private_impl.cc

@@ -1024,6 +1024,7 @@ void DownloadManifestToBuffer(PP_Instance instance,
         FROM_HERE,
         base::Bind(callback.func, callback.user_data,
                    static_cast<int32_t>(PP_ERROR_FAILED)));
+    return;
   }
   const blink::WebDocument& document =
       plugin_instance->GetContainer()->GetDocument();
@@ -1379,6 +1380,7 @@ void PPBNaClPrivate::DownloadNexe(PP_Instance instance,
         FROM_HERE,
         base::Bind(callback.func, callback.user_data,
                    static_cast<int32_t>(PP_ERROR_FAILED)));
+    return;
   }
   const blink::WebDocument& document =
       plugin_instance->GetContainer()->GetDocument();
@@ -1531,6 +1533,7 @@ void DownloadFile(PP_Instance instance,
     base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE, base::Bind(callback, static_cast<int32_t>(PP_ERROR_FAILED),
                               kInvalidNaClFileInfo));
+    return;
   }
   const blink::WebDocument& document =
       plugin_instance->GetContainer()->GetDocument();