shell: fix returncode that can be None in shell

Subprocess return code can be None, so we ensure that shell is returning
a -1 when this happens.

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>

Author: acerv

libkirk/channels/shell.py

@@ -203,7 +203,9 @@ async def run_command(
                 ret = {
                     "command": command,
                     "stdout": stdout,
-                    "returncode": proc.returncode,
+                    "returncode": proc.returncode
+                    if proc.returncode is not None
+                    else -1,
                     "exec_time": t_end,
                 }