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

block/iscsi.c

@@ -1326,7 +1326,7 @@ static int iscsi_open(BlockDriverState *bs, QDict *options, int flags,
         goto out;
     }
 
-    if (iscsi_url->user != NULL) {
+    if (iscsi_url->user[0] != '\0') {
         ret = iscsi_set_initiator_username_pwd(iscsi, iscsi_url->user,
                                               iscsi_url->passwd);
         if (ret != 0) {