fix(cmd_sub): Prevent crash with malformed JID in /sub command

Added null check for jidp in cmd_sub to handle jid_create returning NULL.

Crash occurred when processing malformed JID inputs like @example.com.

Ensures robust handling of invalid JIDs.

Fixes #22

tests/unittests/unittests.c

@@ -386,6 +386,7 @@ main(int argc, char* argv[])
 
         cmocka_unit_test(cmd_sub_shows_message_when_not_connected),
         cmocka_unit_test(cmd_sub_shows_usage_when_no_arg),
+        cmocka_unit_test(test_cmd_sub_malformed_jid),
 
         cmocka_unit_test(contact_in_group),
         cmocka_unit_test(contact_not_in_group),