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

Summary

The `VariantStrIter::impl_get` 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::c_char` pointer initialized to `NULL` was passed as an argument to a C function that that mutates the pointer behind `&p` in-place (i.e. as an out-argument), which was unsound. After changes in recent versions of the Rust compiler, these unsound writes through `&p` now seem to be completely disregarded when building the `glib` crate with optimizations.

This subsequently caused all calls of `VariantStrIter::impl_get` to violate the safety requirements of the `std::ffi::CStr::from_ptr` function - which requires its argument to be a valid pointer to a C-style string - resulting in crashes due to `NULL` pointer dereferences.

This was fixed by passing the out-argument pointer explitly as `&mut p` instead of `&p`.

This issue has been present since this code was initially added in `glib` v0.15.0. The mismatch in mutability was likely missed (and not raised as an error by the compiler) because the C function wrapped by `VariantStrIter::impl_get` is variadic (`glib_sys::g_variant_get_child`), and the pointer in question is one of the variadic arguments.