BUG/MINOR:: mux-h1: Never handle error at mux level for running connection

BUG/MINOR:: mux-h1: Never handle error at mux level for running connection

During the request parsing, we must be sure to never handle errors at the
mux level if the connection is running (or closing). The error must be
handled by the upper layer.

It should never happen. But there is an edge case that was not properly
handled. If all data are received on the first packet with the read0 and the
request is truncated on the payload, in this case the stream-connector is
created, so the H1C is in RUNNING state. But an error is reported because
the request is truncated. In this specific case, the error is handled by the
mux while it should not.

This patch is related to #1966. It must be backported to 2.7.