ostree_sysroot.c: Don't close sysroot_fd twice.

If ostree_sysroot_unload() was called explicitly, then sysroot_fd
would be closed again at finalization time, possibly closing a
random file descriptor belonging to some other part of the
application.

Closes: #507
Approved by: cgwalters

src/libostree/ostree-sysroot.c

@@ -260,7 +260,10 @@ void
 ostree_sysroot_unload (OstreeSysroot  *self)
 {
   if (self->sysroot_fd != -1)
-    (void) close (self->sysroot_fd);
+    {
+      (void) close (self->sysroot_fd);
+      self->sysroot_fd = -1;
+    }
 }
 
 /**