Fix potential UAF in ConcatInPlace.

core/src/fxcrt/fx_basic_wstring_unittest.cpp

 // Copyright 2014 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.
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "../../../testing/fx_string_testhelpers.h"
 #include "../../include/fxcrt/fx_basic.h"
 
 TEST(fxcrt, WideStringOperatorSubscript) {
     // CFX_WideString includes the NUL terminator for non-empty strings.
@@ skipping 231 matching lines @@
     const wchar_t* c_string2 = L"hellp";
     const wchar_t* c_string3 = L"hellod";
     EXPECT_TRUE(wide_string != c_string1);
     EXPECT_TRUE(wide_string != c_string2);
     EXPECT_TRUE(wide_string != c_string3);
     EXPECT_TRUE(c_string1 != wide_string);
     EXPECT_TRUE(c_string2 != wide_string);
     EXPECT_TRUE(c_string3 != wide_string);
 }
 
+TEST(fxcrt, WideStringConcatInPlace) {
+    CFX_WideString fred;
+    fred.ConcatInPlace(4, L"FRED");
+    EXPECT_EQ(L"FRED", fred);
+
+    fred.ConcatInPlace(2, L"DY");
+    EXPECT_EQ(L"FREDDY", fred);
+
+    fred.Delete(3, 3);
+    EXPECT_EQ(L"FRE", fred);
+
+    fred.ConcatInPlace(1, L"D");
+    EXPECT_EQ(L"FRED", fred);
+
+    CFX_WideString copy = fred;
+    fred.ConcatInPlace(2, L"DY");
+    EXPECT_EQ(L"FREDDY", fred);
+    EXPECT_EQ(L"FRED", copy);
+
+    // Test invalid arguments.
+    copy = fred;
+    fred.ConcatInPlace(-6, L"freddy");
+    CFX_WideString not_aliased(L"xxxxxx");
+    EXPECT_EQ(L"FREDDY", fred);
+    EXPECT_EQ(L"xxxxxx", not_aliased);
+}
+
 #define ByteStringLiteral(str) CFX_ByteString(FX_BSTRC(str))
 
 TEST(fxcrt, WideStringUTF16LE_Encode) {
   struct UTF16LEEncodeCase {
     CFX_WideString ws;
     CFX_ByteString bs;
   } utf16le_encode_cases[] = {
     { L"", ByteStringLiteral("\0\0") },
     { L"abc", ByteStringLiteral("a\0b\0c\0\0\0") },
     { L"abcdef", ByteStringLiteral("a\0b\0c\0d\0e\0f\0\0\0") },
@@ skipping 144 matching lines @@
     const wchar_t* c_string2 = L"hellp";
     const wchar_t* c_string3 = L"hellod";
     EXPECT_TRUE(wide_string_c != c_string1);
     EXPECT_TRUE(wide_string_c != c_string2);
     EXPECT_TRUE(wide_string_c != c_string3);
 
     EXPECT_TRUE(c_string1 != wide_string_c);
     EXPECT_TRUE(c_string2 != wide_string_c);
     EXPECT_TRUE(c_string3 != wide_string_c);
 }