libudev: enumerate: fix NULL-deref for subsystem-matches

udev_device_get_subsystem() may return NULL if no subsystem could be
figured out by libudev. This might be due to OOM or if the device
disconnected between the udev_device_new() call and
udev_device_get_subsystem(). Therefore, we need to handle subsystem==NULL
safely.

Instead of testing for it in each helper, we treat subsystem==NULL as
empty subsystem in match_subsystem().

Backtrace of udev_enumerate with an input-device disconnecting in exactly
this time-frame:
 (gdb) bt
 #0  0x00007ffff569dc24 in strnlen () from /usr/lib/libc.so.6
 #1  0x00007ffff56d9e04 in fnmatch@@GLIBC_2.2.5 () from /usr/lib/libc.so.6
 #2  0x00007ffff5beb83d in match_subsystem (udev_enumerate=0x7a05f0, subsystem=0x0) at src/libudev/libudev-enumerate.c:727
 #3  0x00007ffff5bebb30 in parent_add_child (enumerate=enumerate@entry=0x7a05f0, path=<optimized out>) at src/libudev/libudev-enumerate.c:834
 #4  0x00007ffff5bebc3f in parent_crawl_children (enumerate=enumerate@entry=0x7a05f0, path=0x7a56b0 "/sys/devices/<shortened>/input/input97", maxdepth=maxdepth@entry=254) at src/libudev/libudev-enumerate.c:866
 #5  0x00007ffff5bebc54 in parent_crawl_children (enumerate=enumerate@entry=0x7a05f0, path=0x79e8c0 "/sys/devices/<shortened>/input", maxdepth=maxdepth@entry=255) at src/libudev/libudev-enumerate.c:868
 #6  0x00007ffff5bebc54 in parent_crawl_children (enumerate=enumerate@entry=0x7a05f0, path=path@entry=0x753190 "/sys/devices/<shortened>", maxdepth=maxdepth@entry=256) at src/libudev/libudev-enumerate.c:868
 #7  0x00007ffff5bec7df in scan_devices_children (enumerate=0x7a05f0) at src/libudev/libudev-enumerate.c:882
 #8  udev_enumerate_scan_devices (udev_enumerate=udev_enumerate@entry=0x7a05f0) at src/libudev/libudev-enumerate.c:919
 #9  0x00007ffff5df8777 in <random_caller> () at some/file.c:181
libudev: enumerate - do not try to match against an empty subsystem