vhost-user-scsi: Fix memleaks in vus_proc_req()

The 'elem' is allocated memory in vu_queue_pop(), and its memory should be
freed in all error branches after vu_queue_pop().
In addition, in order to free the 'elem' memory outside of while(1) loop, move
the definition of 'elem' to the beginning of vus_proc_req().

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20201125013055.34147-1-alex.chen@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

contrib/vhost-user-scsi/vhost-user-scsi.c

@@ -232,6 +232,7 @@ static void vus_proc_req(VuDev *vu_dev, int idx)
     VugDev *gdev;
     VusDev *vdev_scsi;
     VuVirtq *vq;
+    VuVirtqElement *elem = NULL;
 
     assert(vu_dev);
 
@@ -248,7 +249,6 @@ static void vus_proc_req(VuDev *vu_dev, int idx)
     g_debug("Got kicked on vq[%d]@%p", idx, vq);
 
     while (1) {
-        VuVirtqElement *elem;
         VirtIOSCSICmdReq *req;
         VirtIOSCSICmdResp *rsp;
 
@@ -288,6 +288,7 @@ static void vus_proc_req(VuDev *vu_dev, int idx)
 
         free(elem);
     }
+    free(elem);
 }
 
 static void vus_queue_set_started(VuDev *vu_dev, int idx, bool started)