BUG/MAJOR: mux-h1: Don't eval input data if an error was reported

BUG/MAJOR: mux-h1: Don't eval input data if an error was reported

If an error was already reported on the H1 connection, pending input data
must not be (re)evaluated in h1_process(). Otherwise an unexpected internal
error will be reported, in addition of the first one. And on some
conditions, this may generate an infinite loop because the mux tries to send
an internal error but it fails to do so thus it loops to retry.

This patch should fix the issue #1356. It must be backported to 2.4.