Fix potential UAF in ConcatInPlace.

core/include/fxcrt/fx_string.h

Index: core/include/fxcrt/fx_string.h
diff --git a/core/include/fxcrt/fx_string.h b/core/include/fxcrt/fx_string.h
index a7b9a23e2552483d7f5c8dad5dc131192ca8a4ad..3614cbe0ee644fdefb032d14e16a23dda05331f0 100644
--- a/core/include/fxcrt/fx_string.h
+++ b/core/include/fxcrt/fx_string.h
@@ -389,6 +389,7 @@ protected:
     void					AllocBeforeWrite(FX_STRSIZE nLen);
 
     StringData* m_pData;
+    friend class fxcrt_ByteStringConcatInPlace_Test;
 };
 inline CFX_ByteStringC::CFX_ByteStringC(const CFX_ByteString& src)
 {
@@ -815,6 +816,7 @@ protected:
     void                    AllocCopy(CFX_WideString& dest, FX_STRSIZE nCopyLen, FX_STRSIZE nCopyIndex) const;
 
     StringData* m_pData;
+    friend class fxcrt_WideStringConcatInPlace_Test;
 };
 inline CFX_WideStringC::CFX_WideStringC(const CFX_WideString& src)
 {