9p: fix multiple NULL-pointer-dereferences

Added checks to prevent GPFs from raising.

Link: http://lkml.kernel.org/r/20180727110558.5479-1-tomasbortoli@gmail.com


Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Reported-by:  <syzbot+1a262da37d3bead15c39@syzkaller.appspotmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>