XFA: bound memcpy length in _png_load_bmp_attribute()
XFA: bound memcpy length in _png_load_bmp_attribute()

BUG=466338
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/997273002

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