CPDF_Document::LoadPattern() and friends always have a valid matrix.

core/fpdfapi/fpdf_page/cpdf_pattern.h

 // Copyright 2016 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef CORE_FPDFAPI_FPDF_PAGE_CPDF_PATTERN_H_
 #define CORE_FPDFAPI_FPDF_PAGE_CPDF_PATTERN_H_
 
 #include "core/fxcrt/include/fx_coordinates.h"
 #include "core/fxcrt/include/fx_system.h"
 
 class CPDF_Document;
 class CPDF_Object;
 class CPDF_ShadingPattern;
 class CPDF_TilingPattern;
 
 class CPDF_Pattern {
  public:
   enum PatternType { TILING = 1, SHADING };
 
   virtual ~CPDF_Pattern();
 
   virtual CPDF_TilingPattern* AsTilingPattern() = 0;
   virtual CPDF_ShadingPattern* AsShadingPattern() = 0;
 
   CPDF_Document* document() { return m_pDocument; }
   CPDF_Object* pattern_obj() { return m_pPatternObj; }
   CFX_Matrix* pattern_to_form() { return &m_Pattern2Form; }
-  CFX_Matrix* parent_matrix() { return &m_ParentMatrix; }
+  const CFX_Matrix& parent_matrix() const { return m_ParentMatrix; }
 
  protected:
   CPDF_Pattern(PatternType type,
                CPDF_Document* pDoc,
                CPDF_Object* pObj,
-               const CFX_Matrix* pParentMatrix);
+               const CFX_Matrix& parentMatrix);
 
   const PatternType m_PatternType;
   CPDF_Document* const m_pDocument;
   CPDF_Object* const m_pPatternObj;
   CFX_Matrix m_Pattern2Form;
-  CFX_Matrix m_ParentMatrix;
+  const CFX_Matrix m_ParentMatrix;
 };
 
 #endif  // CORE_FPDFAPI_FPDF_PAGE_CPDF_PATTERN_H_