Fix an assertion failure in CPDFSDK_Widget* Field::GetWidget()
Fix an assertion failure in CPDFSDK_Widget* Field::GetWidget()

BUG=pdfium:291
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1487553003 .

andoma/pdfium,andoma/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,andoma/pdfium,DrAlexx/pdfium,andoma/pdfium,DrAlexx/pdfium