When using libvirt 1.0.5 (maybe earlier, not sure), and an exception is
thrown from the node-libvirt, trying to inspect the "str3" property of
the exception causes a segmentation fault 11 in node. This patches that
problem by first ensuring the "str3" symbol is not null before passing
it to v8::String::New, and adds the same guards to the "str1" and "str2"
symbols as well as a precaution.