BUG: http: do not print garbage on invalid requests in debug mode

BUG: http: do not print garbage on invalid requests in debug mode

Cyril Bonté reported a mangled debug output when an invalid request
was sent with a faulty request line. The reason was the use of the
msg->sl.rq.l offset which was not yet initialized in this case. So
we change the way to report such an error so that first we initialize
it to zero before parsing a message, then we use that to know whether
we can trust it or not. If it's still zero, then we display the whole
buffer, truncated by debug_hdr() to the first CR or LF character, which
results in the first line only.

The same operation was performed for the response, which was wrong too.