rs: Fix a use after free.

[jhb]

Summary

Using a pointer passed to realloc() after realloc() even for pointer
arithmetic is UB. It also breaks in practice on CHERI systems as
the updated value of 'sp' in this case would have had the bounds from
the old allocation.

This would be much cleaner if elem were a std::vector<char *>.

Reported by: GCC -Wuse-after-free

[jhb]

@brooks It is interesting that GCC 12 now warns about this type of UB with realloc().

[brooks]

The "you can't even do math" thing always seems excessive, but if gcc is going to warn on it then at least it will clean up all the CHERI realloc issues.