common/bl: c_str() examines length instead of no. of bptrs.

This approach has 2 adventages:
* it takes into consideration the specical case of a bufferlist
  that carries solely empty bptrs while
* allowing compilers to optimize-out the load-and-check of `_len`
  we already do in `rebuild()` which is called by `c_str()`.

  ```cpp
  void buffer::list::rebuild()
  {
    if (_len == 0) {
      _carriage = &always_empty_bptr;
      _buffers.clear_and_dispose();
      _num = 0;
      return;
    }
    // ...
  }
  ```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>

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