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/test_cmd_sub.h

@@ -1,2 +1,3 @@
 void cmd_sub_shows_message_when_not_connected(void** state);
 void cmd_sub_shows_usage_when_no_arg(void** state);
+void test_cmd_sub_malformed_jid(void** state);