fix: Sanitize account names

GKeyFile parses ini-like config files. So the characters '[', ']',
and '=' cannot be used.
We will replace them with '_' now.

This bug was found when a user wanted to connect to an IPv6 address (user@[ipv6:address]).

One problem could be that we get duplicate account names with they would
use one of those 3 characters. But I consider this very unlikely.

Fix https://github.com/profanity-im/profanity/issues/2103

src/event/server_events.c

@@ -86,6 +86,10 @@ void
 sv_ev_login_account_success(char* account_name, gboolean secured)
 {
     ProfAccount* account = accounts_get_account(account_name);
+    if (!account) {
+        log_error("Could not find account: %s", account_name);
+        return;
+    }
 
     bookmark_ignore_on_connect(account->jid);