bdev: Fix warning about scanbuild error on fedora30.

In file included from bdev_ut.c:43:
/root/yidong/spdk/lib/bdev/bdev.c:4373:9: warning: Access to field 'bdev'
results in a dereference of a null pointer (loaded from variable 'desc')
        return desc->bdev;
               ^~~~~~~~~~

This is related to issue #822.

Change-Id: I8cd2bafadeff9846169bc9ca67b3c4110e9c0da8
Signed-off-by: yidong0635 <dongx.yi@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/459529
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Seth Howell <seth.howell5141@gmail.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>

lib/bdev/bdev.c

@@ -4382,6 +4382,7 @@ spdk_bdev_module_release_bdev(struct spdk_bdev *bdev)
 struct spdk_bdev *
 spdk_bdev_desc_get_bdev(struct spdk_bdev_desc *desc)
 {
+	assert(desc != NULL);
 	return desc->bdev;
 }