block/ssh: fix possible segmentation fault when .desc is not null-terminated

This patch prevents a possible segmentation fault when .desc members are checked
against NULL.

The ssh_runtime_opts was added by commit
8a6a8089 ("block/ssh: Use QemuOpts for runtime
options").

This fix was inspired by
http://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg00883.html

.

Fixes: 8a6a8089 ("block/ssh: Use QemuOpts for runtime options")
Cc: Max Reitz <mreitz@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>