Unsoundness in `Iterator` and `DoubleEndedIterator` impls for `glib::VariantStrIter`

The VariantStrIter::implget function called internally by implementations of the Iterator and DoubleEndedIterator traits for this type was unsound, resulting in undefined behaviour. An immutable reference &p to a mut libc::cchar pointer initialized to NULL was passed as an argument to a C functio...