Crash when packing empty vector<char>

Attempts to pack an empty std::vector<char> result in undefined behavior.

This:

```
msgpack::sbuffer msgBuf;
std::vector<char> data(0);
msgpack::pack(msgBuf, data);
```

ultimately calls data.front():

```
//vector_char.hpp:54
o.pack_bin_body(&v.front(), size);
```

which is only valid on a vector of nonzero length.
