libvhost-user: check for NULL when allocating a virtqueue element

libvhost-user: check for NULL when allocating a virtqueue element

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>
Message-Id: <20230210112514.16858-1-clopez@suse.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>