fix invalid free of duplocale object when malloc has been replaced

fix invalid free of duplocale object when malloc has been replaced

newlocale and freelocale use __libc_malloc and __libc_free, but
duplocale used malloc. If malloc was replaced, this resulted in
invalid free using the wrong allocator when passing the result of
duplocale to freelocale.

Instead, use libc-internal malloc for duplocale.

This bug was introduced by commit
1e4204d522670a1d8b8ab85f1cfefa960547e8af.