OutOfProcessEngine::ScrollToPage(): check for null engine

This callback may be reached as part of a close document action
after the engine has been destroyed. Further inspection of the code
shows that the same might also happen for Print().

Bug: 901640
Change-Id: I8faaeaf72adb37441f9e9c977093273c9e055e91
Reviewed-on: https://chromium-review.googlesource.com/c/1320810Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605829}

pdf/out_of_process_instance.cc

@@ -1385,7 +1385,7 @@ void OutOfProcessInstance::ScrollBy(const pp::Point& point) {
 }
 
 void OutOfProcessInstance::ScrollToPage(int page) {
-  if (engine_->GetNumberOfPages() == 0)
+  if (!engine_ || engine_->GetNumberOfPages() == 0)
     return;
 
   pp::VarDictionary message;
@@ -1582,8 +1582,9 @@ void OutOfProcessInstance::Email(const std::string& to,
 }
 
 void OutOfProcessInstance::Print() {
-  if (!engine_->HasPermission(PDFEngine::PERMISSION_PRINT_LOW_QUALITY) &&
-      !engine_->HasPermission(PDFEngine::PERMISSION_PRINT_HIGH_QUALITY)) {
+  if (!engine_ ||
+      (!engine_->HasPermission(PDFEngine::PERMISSION_PRINT_LOW_QUALITY) &&
+       !engine_->HasPermission(PDFEngine::PERMISSION_PRINT_HIGH_QUALITY))) {
     return;
   }