BUG/MINOR: mux-fcgi: Handle empty STDERR record

BUG/MINOR: mux-fcgi: Handle empty STDERR record

As mentionned in the FastCGI specification, FCGI "streams" are series of
non-empty stream records (length != 0), followed by an empty one. It is properly
handled for FCGI_STDOUT records, but not for FCGI_STDERR ones. If an empty
FCGI_STDERR record is received, the connection is blocked waiting for data which
will never come.

To fix the bug, when an empty FCGI_STDERR record is received, we drop it, eating
the padding if any.

This patch should fix the issue #743. It must be backported as far as 2.1.