FPDFPage_HasFormFieldAtPoint should not crash for nullptr pages.
FPDFPage_HasFormFieldAtPoint should not crash for nullptr pages.

BUG=pdfium:401
R=jun_fang@foxitsoftware.com

Review URL: https://codereview.chromium.org/1713833003 .

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