BUG/MEDIUM: log: fix risk of segfault when logging HTTP fields in TCP mode

BUG/MEDIUM: log: fix risk of segfault when logging HTTP fields in TCP mode

David Torgerson faced an issue when using HTTP fields in log-format in TCP
sections. The txn is dereferenced while it's null, resulting in a crash of
the process. Such configurations are invalid and a warning is emitted, but
nevertheless the process must not crash. As found by Lukas Tribus, this is
a side effect of the split between the stream and the HTTP transaction that
happened in 1.6, making it possible to have txn==NULL there.

The fix consists in checking that txn is valid before using it. Fortunately
it's easy since almost all places already used to check for the existence
of a field (eg: txn->uri).

This patch should be backported to 1.6.