Fixed bug reported by Julian Foad:

While investigating, I found that SGPropertyNode::getPath returns a
(char *) pointer to the character data of a string on its stack,
i.e. to undefined memory after it returns.  I remember someone was
changing strings to char* for efficiency.  Perhaps this bug was
introduced then.  I'll include a patch for it with my eventual patch
for the above, unless someone beats me to it.  I don't think it
affects any existing callers: they all want a string anyway.