wayland: unlink shm file earlier in create function

create_shm_pool unlinks the temporary file a little,
too late. It should be unlinked before ftruncate()
is called for two reasons:

1) if ftruncate fails, the file is currently not
getting cleaned up at all
2) in theory, if the file is public some other process
could muck with it

This commit just moves the unlink call a little higher
up.

https://bugzilla.gnome.org/show_bug.cgi?id=760897

gdk/wayland/gdkdisplay-wayland.c

@@ -941,6 +941,7 @@ create_shm_pool (struct wl_shm  *shm,
                   filename, g_strerror (errno));
       return NULL;
     }
+  unlink (filename);
 
   stride = width * 4;
   size = stride * height;
@@ -953,7 +954,6 @@ create_shm_pool (struct wl_shm  *shm,
     }
 
   data = mmap (NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
-  unlink (filename);
 
   if (data == MAP_FAILED)
     {