Bug 1383501: Assert that com proxy is non-null before calling SendPDocAccessibleConstructor; r=yzen

MozReview-Commit-ID: CuIXyoe4jsj

accessible/generic/DocAccessible.cpp

@@ -1503,6 +1503,7 @@ DocAccessible::DoInitialUpdate()
 
 #if defined(XP_WIN)
         IAccessibleHolder holder(CreateHolderFromAccessible(this));
+        MOZ_DIAGNOSTIC_ASSERT(!holder.IsNull());
         int32_t childID = AccessibleWrap::GetChildIDFor(this);
 #else
         int32_t holder = 0, childID = 0;