qemu-sockets: fix unix socket path copy (again)

qemu-sockets: fix unix socket path copy (again)

Commit 4cfd970ec188558daa6214f26203fe553fb1e01f added an
assert which ensures the path within an address of a unix
socket returned from the kernel is at least one byte and
does not exceed sun_path buffer. Both of this constraints
are wrong:

A unix socket can be unnamed, in this case the path is
completely empty (not even \0)

And some implementations (notable linux) can add extra
trailing byte (\0) _after_ the sun_path buffer if we
passed buffer larger than it (and we do).

So remove the assertion (since it causes real-life breakage)
but at the same time fix the usage of sun_path. Namely,
we should not access sun_path[0] if kernel did not return
it at all (this is the case for unnamed sockets),
and use the returned salen when copyig actual path as an
upper constraint for the amount of bytes to copy - this
will ensure we wont exceed the information provided by
the kernel, regardless whenever there is a trailing \0
or not. This also helps with unnamed sockets.

Note the case of abstract socket, the sun_path is actually
a blob and can contain \0 characters, - it should not be
passed to g_strndup and the like, it should be accessed by
memcpy-like functions.

Fixes: 4cfd970ec188558daa6214f26203fe553fb1e01f
Fixes: http://bugs.debian.org/993145
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
CC: qemu-stable@nongnu.org