PDF: Fix a crash from r372196 with the print preview PDFEngine client.

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 <stddef.h>
 #include <stdint.h>
 
@@ skipping 590 matching lines @@
   IPDF_JSPLATFORM::Doc_print = Form_Print;
   IPDF_JSPLATFORM::Doc_submitForm = Form_SubmitForm;
   IPDF_JSPLATFORM::Doc_gotoPage = Form_GotoPage;
   IPDF_JSPLATFORM::Field_browse = Form_Browse;
 
   IFSDK_PAUSE::version = 1;
   IFSDK_PAUSE::user = NULL;
   IFSDK_PAUSE::NeedToPauseNow = Pause_NeedToPauseNow;
 
 #if defined(OS_LINUX)
-  g_last_instance_id = client_->GetPluginInstance()->pp_instance();
+  // PreviewModeClient does not know its pp::Instance.
+  pp::Instance* instance = client_->GetPluginInstance();
+  if (instance)
+    g_last_instance_id = instance->pp_instance();
 #endif
 }
 
 PDFiumEngine::~PDFiumEngine() {
   for (auto& page : pages_)
     page->Unload();
 
   if (doc_) {
     FORM_DoDocumentAAction(form_, FPDFDOC_AACTION_WC);
 
@@ skipping 3326 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