BUG/MEDIUM: connection: Ignore PP2 unique ID for stream-less connections

BUG/MEDIUM: connection: Ignore PP2 unique ID for stream-less connections

It is possible to send a unique ID when the PROXY protocol v2 is used. It relies
on the stream to do so. So we must be sure to have a stream. Locally initiated
connections may not be linked to a stream. For instance, outgoing connections
created by health checks have no stream. Moreover, the stream is not retrieved
for mux-less connections (this bug will be fixed in another commit).

Unfortunately, in make_proxy_line_v2() function, the stream is not tested before
generating the unique-id.  This bug leads to a segfault when a health check is
performed for a server with the PROXY protocol v2 and the unique-id option
enabled. It also crashes for servers using SSL connections with alpn. The bug
was introduced by the commit cf6e0c8a8 ("MEDIUM: proxy_protocol: Support sending
unique IDs using PPv2")

This patch should fix the issue #640. It must be backported to the same versions
as the commit above.