The usage of "+= snprintf" in print_preamble_header()/print_preamble() is wrong.

https://github.com/emilk/loguru/blob/bead38889d44d9fdb5c52916d1f26c4d6af09e66/loguru.hpp#L2329

Ref: http://www.cplusplus.com/reference/cstdio/snprintf/

1. The type of argument `buf_size` is `size_t`, but `out_buff_size - pos` may produce negative value.
2. The return value maybe negative if an encoding error occurs.
3. If `out_buff_size == 0`, there is no `\0`.