[Bug libstdc++/58415] __sso_string_base move constructor does not null terminate local data when moving from empty string

["mkirzinger at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58415

--- Comment #2 from Michael Kirzinger <mkirzinger at gmail dot com> ---
There appears to be one additional problem: if __rcs._M_is_local() is true, but
__rcs._M_length() is false, the buffer of the string being created is never
null terminated/zeroed.

Example:
----------------------------------------------------------
#include <ext/vstring.h>
#include <cstdio>
#include <cstdlib>
#include <cstring>

typedef __gnu_cxx::__versa_string<char> string;

int main()
{
    char buf[sizeof(string)+1] = "stringstringstring";

    string s1;
    string* s2 = new (buf) string(std::move(s1));

    printf("%s\n", s2->c_str());
}