mdev: Fix mingw build by adding a check for non-NULL pointer

mdev: Fix mingw build by adding a check for non-NULL pointer

This patch fixes the following MinGW error (although actually being a
false positive):

../../src/util/virmdev.c: In function 'virMediatedDeviceListMarkDevices':
../../src/util/virmdev.c:453:21: error: potential null pointer
dereference [-Werror=null-dereference]
          const char *mdev_path = mdev->path;
                      ^~~~~~~~~

Signed-off-by: Erik Skultety <eskultet@redhat.com>