iscsi: Fix check for username

The variable user in struct iscsi_url is a character array, not a pointer.
Therefore its address will never be NULL.

clang reports this error:

block/iscsi.c:1329:20: warning:
 comparison of array 'iscsi_url->user' not equal to a null pointer
 is always true [-Wtautological-pointer-compare]

Reviewed-by: Peter Lieven <pl@kamp.de>
Acked-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <1425719670-5486-1-git-send-email-sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
