BUG/MINOR: http_ana: Report -1 for %Tr for invalid response only

BUG/MINOR: http_ana: Report -1 for %Tr for invalid response only

The server response time is erroneously reported as -1 when it is
intercepted by HAProxy.

As stated in the documentation, the server response time is reported as -1
when the last response header was never seen. It happens when a server
timeout is triggered before the server managed to process the request. It
also happens if the response is invalid. This may be reported by the mux
during the response parsing, but also by the HTTP analyzers. However, in
this last case, the response time must only be reported as -1 on 502.

This patch must be backported to all stable versions. It should fix the
issue #2384.