Update ensure_byte_buf_has_allocated_buffer_member to include primary allocation

Update `ensure_byte_buf_has_allocated_buffer_member` to include primary allocation so that the code
```
struct aws_byte_buf *buf1 = malloc(sizeof(*buf1));

ensure_byte_buf_has_allocated_buffer_member(buf1);
```
can be replaced by
```
struct aws_byte_buf *buf1 = ensure_byte_buf_has_allocated_buffer_member(buf1);
```
