[RFC PATCH guest-common] tunnel: treat reader_stderr as optional parameter in read_tunnel

[Daniel Kral <d.kral@proxmox.com>]

The reader_stderr parameter was introduced for a specific use case for
the users of fork_ssh_tunnel(). The other callers, such as
fork_websocket_tunnel(), do not provide the parameter and the Perl
interpreter will warn about this:

    Use of uninitialized value $reader_stderr in <HANDLE> at /usr/share/perl5/PVE/Tunnel.pm line 77.
    readline() on unopened filehandle at /usr/share/perl5/PVE/Tunnel.pm line 77.

Therefore, treat reader_stderr as an optional parameter by not reading
from the handle if it is not provided.

Fixes: 8c8aa63 ("tunnel: redirect stderr to log function")
Signed-off-by: Daniel Kral <d.kral@proxmox.com>
---
 src/PVE/Tunnel.pm | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/PVE/Tunnel.pm b/src/PVE/Tunnel.pm
index 148a8e2..68547a5 100644
--- a/src/PVE/Tunnel.pm
+++ b/src/PVE/Tunnel.pm
@@ -74,8 +74,10 @@ sub read_tunnel {
     };
     my $err = $@;
 
-    while (my $line = <$reader_stderr>) { # $reader_stderr is set up as non-blocking
-        $tunnel->{log}->('warn', $line);
+    if (defined($reader_stderr)) {
+        while (my $line = <$reader_stderr>) { # $reader_stderr is set up as non-blocking
+            $tunnel->{log}->('warn', $line);
+        }
     }
 
     die "reading from tunnel failed: $err\n" if $err;
-- 
2.47.3