vhost: fix potential null pointer dereference

Fix the potential NULL pointer dereference issue raised by Coverity.

    578             reconn = malloc(sizeof(*reconn));
    >>>     CID 127481:  Null pointer dereferences  (NULL_RETURNS)
    >>>     Dereferencing a null pointer "reconn".
    579             reconn->un = un;

Coverity issue: 127481
Fixes: e623e0c6d8a5 ("vhost: add reconnect ability")

Reported-by: John McNamara <john.mcnamara@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>