We should check that NULL is not being passed for String parameters. At the moment if a program does this, it probably causes a segfault. The calling program is technically incorrect, but we should check for this case anyway, particularly since in some bindings (eg. Perl) an undefined string parameter might be passed as NULL.
Moving upstream, since it's only a safety check for incorrect programs
Patch posted upstream: https://www.redhat.com/archives/libguestfs/2010-May/msg00117.html