Clear parent symbol references when flattening derived library symbols.

Broken parent symbols could lead to invalid return values and flattened
library symbols cannot be derived from a parent symbol.

(cherry picked from commit 360d185e3b)

eeschema/lib_symbol.cpp

@@ -669,6 +669,7 @@ std::unique_ptr< LIB_SYMBOL > LIB_SYMBOL::Flatten() const
         retv->SetExcludedFromBoard( parent->GetExcludedFromBoard() );
 
         retv->UpdateFieldOrdinals();
+        retv->m_parent.reset();
     }
     else
     {