BUG/MINOR: mux-h1: avoid copying output over itself in zero-copy

BUG/MINOR: mux-h1: avoid copying output over itself in zero-copy

It's almost funny but one side effect of the latest zero-copy changes made
to mux-h1 resulted in the temporary buffer being copied over itself at the
exact same location. This has no impact except slowing down operations and
irritating valgrind. The cause is an incorrect pointer check after the
alignment optimizations were made.

This needs to be backported to 1.9.

Reported-by: Tim Duesterhus <tim@bastelstu.be>