chat: fix segfault when domain is not provided

Running xmppipe in chat mode without a full jid:

    xmppipe --chat --output foo

Caused a segfault when a NULL domain was passed to
xmpp_stanza_set_attribute(). The libstrophe functions do not check for
NULL and so crash calling strlen(NULL).

Set the jid's domain from the user's username. If the user's jid is
user@example.com:

    # expanded to foo@example.com
    xmppipe --chat --output foo

src/xmppipe.c

@@ -270,6 +270,13 @@ main(int argc, char **argv)
         state->out = xmppipe_strdup(state->room);
         state->mucjid = xmppipe_mucjid(state->out, state->resource);
     }
+    else if (!(state->opt & XMPPIPE_OPT_GROUPCHAT)) {
+        char *from = strchr(jid, '@');
+        if (from == NULL)
+            usage(state);
+        from++;
+        state->out = xmppipe_conference(state->room, from);
+    }
 
     if (xmppipe_fmt_init() < 0)
         errx(EXIT_FAILURE, "xmppipe_fmt_init");