linux_udev: Initialize pointer before calling external API

linux_udev: Initialize pointer before calling external API

udev_list_entry_foreach() sets the entry variable for each device but
initializing the pointer to NULL to avoid warnings.

Found by Coverity:

libusb-1.0.21/libusb/os/linux_udev.c:273:
error[uninitvar]: Uninitialized variable: entry
>   271|    devices = udev_enumerate_get_list_entry(enumerator);
>   272|
>   273|->  udev_list_entry_foreach(entry, devices) {
>   274|        const char *path = udev_list_entry_get_name(entry);
>   275|        uint8_t busnum = 0, devaddr = 0;

Signed-off-by: Victor Toso <victortoso@redhat.com>
Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>