With JSON_DIAGNOSTIC_POSITIONS, comparing a value-initialized iterator
(which has no container) with another iterator built the
invalid_iterator.212 message from the byte positions of the iterator's
value, which is a null pointer: UBSan reports "member call on null
pointer" in exception::get_byte_positions(), then the process crashes.
Every other configuration throws invalid_iterator.212.
get_byte_positions() now returns no positions for a null value, as the
JSON_DIAGNOSTICS path already does.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
0eabcc51eb