Return no style in GetItemStyles() for item of null pointer
Return no style in GetItemStyles() for item of null pointer

The error handling in GetItemStyles() of list box is wrong. -1 is
0xFFFFFFFF in uint32_t, which suggests all the style bits on. This was
discovered by a signed/unsigned mismatch warning.

BUG=pdfium:29

Review-Url: https://codereview.chromium.org/2005583002

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