undefined behavior in do_fill_uninitialized_memory()

Using gcc 9.1 with -fsanitize=undefined and -DBOOST_CB_ENABLE_DEBUG=1, the following code results in a ubsan error: "boost/circular_buffer/debug.hpp:37:16: runtime error: null pointer passed as argument 1, which is declared to never be null".

```
using Q = boost::circular_buffer;
Q q;
const Q q2(q);
```

The ubsan error is a result of the following code in circular_buffer/debug.h:

```
template
inline void do_fill_uninitialized_memory(T* data, std::size_t size_in_bytes) BOOST_NOEXCEPT {
std::memset(static_cast(data), UNINITIALIZED, size_in_bytes);
}
```

During copy construction, the function gets called with data == nullptr and size_in_bytes == 0. I believe that passing a null pointer to memset is technically undefined behavior even if the size is 0.

Changing the above function as follows avoids the ubsan error:

```
template
inline void do_fill_uninitialized_memory(T* data, std::size_t size_in_bytes) BOOST_NOEXCEPT {
if (size_in_bytes != 0u) {
std::memset(static_cast(data), UNINITIALIZED, size_in_bytes);
}
}
```

I had originally written it to check for data != nullptr, but since this is debug code I thought it seemed desirable to know if the function is ever called with data == null and size_in_bytes != 0. In any case, either way will prevent the ubsan error.