[Libguestfs] [PATCH nbdkit 0/2] Fix a couple of problems found by Coverity.

There are a few other issues that Coverity found, but I believe all can be ignored ... except one: We don't set umask anywhere inside nbdkit. Coverity complains that this is a problem where we create temporary files, since the result of mkstemp depends implicitly on the umask value. I think we might consider setting umask anyway (eg. to 022) just to make plugin behaviour more predictable. What do you think? Rich.