Fix crash in PDF Viewer when clicking on link to an invalid page.

Bug: 821301
Change-Id: I573f5f90fa896f92f91ca63bfda2588469ab5aab
Reviewed-on: https://chromium-review.googlesource.com/960594Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542864}

pdf/pdfium/pdfium_engine.cc

@@ -2030,6 +2030,9 @@ bool PDFiumEngine::OnMouseUp(const pp::MouseInputEvent& event) {
       return true;
     }
     if (area == PDFiumPage::DOCLINK_AREA) {
+      if (!PageIndexInBounds(target.page))
+        return true;
+
       pp::Rect page_rect(GetPageScreenRect(target.page));
       int y = position_.y() + page_rect.y();
       if (target.y_in_pixels)