BUG/MEDIUM: http: don't dump debug headers on MSG_ERROR

BUG/MEDIUM: http: don't dump debug headers on MSG_ERROR

When the HTTP parser is in state HTTP_MSG_ERROR, we don't know if it was
already initialized or not. If the error happens before HTTP_MSG_RQBEFORE,
random offsets might be present and we don't want to display such random
strings in debug mode.

While it's theorically possible to randomly crash the process when running
in debug mode here, this bug was not tagged MAJOR because it would not
make sense to run in debug mode in production.

This fix must be backported to 1.5 and 1.4.