util/ioc.c: try to reassure Coverity about qemu_iovec_init_extended

Make it more obvious, that filling qiov corresponds to qiov allocation,
which in turn corresponds to total_niov calculation, based on mid_niov
(not mid_len). Still add an assertion to show that there should be no
difference.

[Added mingw "error: 'mid_iov' may be used uninitialized in this
function" compiler error fix suggested by Vladimir.
--Stefan]

Reported-by: Coverity (CID 1405302)
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190910090310.14032-1-vsementsov@virtuozzo.com
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190910090310.14032-1-vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

fixup! util/ioc.c: try to reassure Coverity about qemu_iovec_init_extended

util/iov.c

@@ -423,7 +423,7 @@ void qemu_iovec_init_extended(
 {
     size_t mid_head, mid_tail;
     int total_niov, mid_niov = 0;
-    struct iovec *p, *mid_iov;
+    struct iovec *p, *mid_iov = NULL;
 
     if (mid_len) {
         mid_iov = qiov_slice(mid_qiov, mid_offset, mid_len,
@@ -446,7 +446,8 @@ void qemu_iovec_init_extended(
         p++;
     }
 
-    if (mid_len) {
+    assert(!mid_niov == !mid_len);
+    if (mid_niov) {
         memcpy(p, mid_iov, mid_niov * sizeof(*p));
         p[0].iov_base = (uint8_t *)p[0].iov_base + mid_head;
         p[0].iov_len -= mid_head;