selftest: net: ovpn: Fix uninit return values

Fix functions that return undefined values. These issues were caught by
running clang using LLVM=1 option.

Clang warnings are as follows:
ovpn-cli.c:1587:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
 1587 |         if (!sock) {
      |             ^~~~~
ovpn-cli.c:1635:9: note: uninitialized use occurs here
 1635 |         return ret;
      |                ^~~
ovpn-cli.c:1587:2: note: remove the 'if' if its condition is always false
 1587 |         if (!sock) {
      |         ^~~~~~~~~~~~
 1588 |                 fprintf(stderr, "cannot allocate netlink socket\n");
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1589 |                 goto err_free;
      |                 ~~~~~~~~~~~~~~
 1590 |         }
      |         ~
ovpn-cli.c:1584:15: note: initialize the variable 'ret' to silence this warning
 1584 |         int mcid, ret;
      |                      ^
      |                       = 0
ovpn-cli.c:2107:7: warning: variable 'ret' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
 2107 |         case CMD_INVALID:
      |              ^~~~~~~~~~~
ovpn-cli.c:2111:9: note: uninitialized use occurs here
 2111 |         return ret;
      |                ^~~
ovpn-cli.c:1939:12: note: initialize the variable 'ret' to silence this warning
 1939 |         int n, ret;
      |                   ^
      |

Fixes: 959bc33 ("testing/selftests: add test tool and scripts for ovpn module")
Signed-off-by: Sidharth Seela <sidharthseela@gmail.com>
Link: https://patch.msgid.link/20251001123107.96244-2-sidharthseela@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: sidharth-458

tools/testing/selftests/net/ovpn/ovpn-cli.c

@@ -1586,6 +1586,7 @@ static int ovpn_listen_mcast(void)
 	sock = nl_socket_alloc();
 	if (!sock) {
 		fprintf(stderr, "cannot allocate netlink socket\n");
+		ret = -ENOMEM;
 		goto err_free;
 	}
 
@@ -2105,6 +2106,7 @@ static int ovpn_run_cmd(struct ovpn_ctx *ovpn)
 		ret = ovpn_listen_mcast();
 		break;
 	case CMD_INVALID:
+		ret = -EINVAL;
 		break;
 	}