pp::Buffer_Dev: Clear PP_Resource upon Map() error in Init()

Now if Map() failed, we clear |data_| and |size_|, but |pp_resource_| is
not cleared. Therefore, if the caller calls buffer.is_null() it will
return false. Since the Init() failed, buffer.is_null() should return
true.

In this CL, if Map() failed, we call Clear() to explicitly clear
|pp_resource_|.

BUG=679801

Review-Url: https://codereview.chromium.org/2629543002
Cr-Commit-Position: refs/heads/master@{#443159}

ppapi/cpp/dev/buffer_dev.cc

@@ -64,6 +64,8 @@ void Buffer_Dev::Init() {
     if (data_)
       return;
   }
+
+  Clear();
   data_ = NULL;
   size_ = 0;
 }