Add CFX_ByteStringC::CharAt() to avoid c_str() and casts

core/fpdfapi/fpdf_font/cpdf_font.cpp

Index: core/fpdfapi/fpdf_font/cpdf_font.cpp
diff --git a/core/fpdfapi/fpdf_font/cpdf_font.cpp b/core/fpdfapi/fpdf_font/cpdf_font.cpp
index f143bed541dbfd6a10624a0b9703611a1a71d524..9fb54a828bec2b2eeedc6803433b835aaeb2ff09 100644
--- a/core/fpdfapi/fpdf_font/cpdf_font.cpp
+++ b/core/fpdfapi/fpdf_font/cpdf_font.cpp
@@ -149,7 +149,7 @@ FX_BOOL CPDF_Font::IsVertWriting() const {
 }
 
 int CPDF_Font::AppendChar(FX_CHAR* buf, uint32_t charcode) const {
-  *buf = (FX_CHAR)charcode;
+  *buf = static_cast<FX_CHAR>(charcode);
   return 1;
 }