check for valid pointer on all returns from aug_get()
    
    Several calls to aug_get() assumed that if the return value wasn't <0,
    that it was returning a valid pointer to a string. Aside from the fact
    that a return of 0 indicates no string, there is also the problem that
    even if the return value is 1, the string could be NULL.
    
    In most cases this didn't cause a problem in practice, because the
    path being sent to aug_get() was a path previously returned by
    aug_match() (implying that it really does point to a value).