[Libguestfs] [PATCH nbdkit v2] common/bitmap: Don't fail on realloc (ptr, 0)

v1 was here: https://www.redhat.com/archives/libguestfs/2019-September/msg00100.html In v2 I've changed the patch so it avoids calling realloc at all in this case. The patch is a bit longer this way. But I don't see any other alternative if we are to avoid having a "realloc wrapper" of some kind that we use everywhere, which I guess we should avoid because it makes plugins just that bit harder to write and we want them to be as easy to write and accessible as possible. Also: - Clean up the commit message. - Add a simple regression test. Rich.