fix(cmd_caps): Prevent crash in /caps with malformed JID input

- Added null check for `jid` in `cmd_caps` to handle `jid_create` returning NULL.
- Crash occurred when processing malformed inputs like `@example.com` in `/caps`.
- Ensures robust handling of invalid JID strings in `WIN_CHAT` and `WIN_CONSOLE` cases.

Fixes #20

src/command/cmd_funcs.c

@@ -3418,7 +3418,7 @@ cmd_caps(ProfWin* window, const char* const command, gchar** args)
         if (args[0]) {
             auto_jid Jid* jid = jid_create(args[0]);
 
-            if (jid->fulljid == NULL) {
+            if (jid == NULL || jid->fulljid == NULL) {
                 cons_show("You must provide a full jid to the /caps command.");
             } else {
                 PContact pcontact = roster_get_contact(jid->barejid);