gwinhttpvfs: Handle g_get_prgname() returning NULL
When prgname wasn't set NULL would be passed to g_utf8_to_utf16() resulting in "g_utf8_to_utf16: assertion 'str != NULL' failed"
When prgname wasn't set NULL would be passed to g_utf8_to_utf16() resulting in "g_utf8_to_utf16: assertion 'str != NULL' failed"