[media] vb2: Fix error handling in '__vb2_buf_mem_alloc'

'call_ptr_memop' can return NULL, so we must test its return value with
'IS_ERR_OR_NULL'. Otherwise, the test 'if (mem_priv)' is meaningless.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>