If we ran out of DMA pool buffers, we get into the unmap
code path with a NULL before. Deal with this by checking
the virtual mapping is not NULL.

Cc: stable@vger.kernel.org
Signed-off-by: NGilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>