Don't return uninitialized memory from PDFiumEngine::Form_Response().

pdf/pdfium/pdfium_engine.cc

 // Copyright (c) 2012 The Chromium 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 "pdf/pdfium/pdfium_engine.h"
 
 #include <math.h>
 
 #include "base/json/json_writer.h"
 #include "base/logging.h"
@@ skipping 3053 matching lines @@
                                 int length) {
   std::string question_str = base::UTF16ToUTF8(
       reinterpret_cast<const base::char16*>(question));
   std::string default_str = base::UTF16ToUTF8(
       reinterpret_cast<const base::char16*>(default_response));
 
   PDFiumEngine* engine = static_cast<PDFiumEngine*>(param);
   std::string rv = engine->client_->Prompt(question_str, default_str);
   base::string16 rv_16 = base::UTF8ToUTF16(rv);
   int rv_bytes = rv_16.size() * sizeof(base::char16);
-  if (response && rv_bytes <= length)
-    memcpy(response, rv_16.c_str(), rv_bytes);
+  if (response) {
+    int bytes_to_copy = rv_bytes < length ? rv_bytes : length;
+    memcpy(response, rv_16.c_str(), bytes_to_copy);
+  }
   return rv_bytes;
 }
 
 int PDFiumEngine::Form_GetFilePath(IPDF_JSPLATFORM* param,
                                    void* file_path,
                                    int length) {
   PDFiumEngine* engine = static_cast<PDFiumEngine*>(param);
   std::string rv = engine->client_->GetURL();
   if (file_path && rv.size() <= static_cast<size_t>(length))
     memcpy(file_path, rv.c_str(), rv.size());
@@ skipping 309 matching lines @@
     double* height) {
   FPDF_DOCUMENT doc = FPDF_LoadMemDocument(pdf_buffer, pdf_buffer_size, NULL);
   if (!doc)
     return false;
   bool success = FPDF_GetPageSizeByIndex(doc, page_number, width, height) != 0;
   FPDF_CloseDocument(doc);
   return success;
 }
 
 }  // namespace chrome_pdf