The compiler complained about assigning the "const char *" returned by
mb_cstring() to a "char *". The warning is correct, as the returned
buffer still belongs to the membuffer. The code only worked because
destruction of the membuffer was "forgotten".
Fix this by using the "detach_buffer()" function, which passes ownership
to the caller and accordingly returns a "char *".
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
