Fix multivolume archive creation when volume length=record size.

* src/tar.c (decode_options): Do not allow volume length less
than record size.
* src/buffer.c (_gnu_flush_write): Compensate for the effect
of eventual flush_archive occurring in the middle of buffer
move.
Increment records_written only if _flush_write was able to write
something.
* tests/multiv06.at: New testcase.
* tests/Makefile.am, test/testsuite.at: Add tests/multiv06.at