Check the return value for malloc(), avoiding a NULL pointer
dereference, and propagate error in function callers.

Found with GCC 13 and -fanalyzer:

../subprojects/libvhost-user/libvhost-user.c: In function ‘virtqueue_alloc_element’:
../subprojects/libvhost-user/libvhost-user.c:2556:19: error: dereference of possibly-NULL ‘elem’ [CWE-690] [-Werror=analyzer-possible-null-dereference]
 2556 |     elem->out_num = out_num;
      |     ~~~~~~~~~~~~~~^~~~~~~~~
  ‘virtqueue_alloc_element’: event 1
    |
    | 2554 |     assert(sz >= sizeof(VuVirtqElement));
    |      |     ^~~~~~
    |      |     |
    |      |     (1) following ‘true’ branch (when ‘sz > 31’)...
    |
  ‘virtqueue_alloc_element’: events 2-4
    |
    | 2555 |     elem = malloc(out_sg_end);
    |      |     ^~~~   ~~~~~~~~~~~~~~~~~~
    |      |     |      |
    |      |     |      (3) this call could return NULL
    |      |     (2) ...to here
    | 2556 |     elem->out_num = out_num;
    |      |     ~~~~~~~~~~~~~~~~~~~~~~~
    |      |                   |
    |      |                   (4) ‘elem’ could be NULL: unchecked value from (3)
    |

Signed-off-by: Carlos López <clopez@suse.de>
---
 subprojects/libvhost-user/libvhost-user.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c
index fc69783d2b..225bbbe3c8 100644
--- a/subprojects/libvhost-user/libvhost-user.c
+++ b/subprojects/libvhost-user/libvhost-user.c
@@ -2553,6 +2553,10 @@ virtqueue_alloc_element(size_t sz,
 
     assert(sz >= sizeof(VuVirtqElement));
     elem = malloc(out_sg_end);
+    if (!elem) {
+        DPRINT("%s: failed to malloc virtqueue element\n", __func__);
+        return NULL;
+    }
     elem->out_num = out_num;
     elem->in_num = in_num;
     elem->in_sg = (void *)elem + in_sg_ofs;
@@ -2639,6 +2643,9 @@ vu_queue_map_desc(VuDev *dev, VuVirtq *vq, unsigned int idx, size_t sz)
 
     /* Now copy what we have collected and mapped */
     elem = virtqueue_alloc_element(sz, out_num, in_num);
+    if (!elem) {
+        return NULL;
+    }
     elem->index = idx;
     for (i = 0; i < out_num; i++) {
         elem->out_sg[i] = iov[i];
-- 
2.35.3