Avoid crashing on CPDF_ToUnicodeMap::Load by using ValueOrDefault()

core/fpdfapi/font/font_int.h

Index: core/fpdfapi/font/font_int.h
diff --git a/core/fpdfapi/font/font_int.h b/core/fpdfapi/font/font_int.h
index 10c2dcada451d20785f0561207466fa949aa551c..02131eb67cd2c3db826a95da72f0674a4bd3c38c 100644
--- a/core/fpdfapi/font/font_int.h
+++ b/core/fpdfapi/font/font_int.h
@@ -196,6 +196,8 @@ class CPDF_ToUnicodeMap {
   static uint32_t StringToCode(const CFX_ByteStringC& str);
   static CFX_WideString StringToWideString(const CFX_ByteStringC& str);
 
+  uint32_t GetUnicode();
+
   std::map<uint32_t, uint32_t> m_Map;
   CPDF_CID2UnicodeMap* m_pBaseMap;
   CFX_WideTextBuf m_MultiCharBuf;