memif: fix uninitialized pointer read coverity error

Set the content of tmp.sock prior to calling memif_msg_send_disconnect.
Also fix the problem socket was not close in the same spot due to
error encountered.

Change-Id: I8f54ebad2250d1944afcc52e71d2a59da05362af
Signed-off-by: Steven <sluong@cisco.com>