reject unreasonably long mailbox names from IMAP LIST

this wasn't really a security problem, as the name mapping we actually
do does not change the string length, and the iteration was already
safe after the literal length fix, but it's still better to catch weird
input.

src/drv_imap.c

@@ -1439,6 +1439,10 @@ parse_list_rsp_p2( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 	}
 	arg = list->val;
 	argl = (int)list->len;
+	if (argl > 1000) {
+		warn( "IMAP warning: ignoring unreasonably long mailbox name '%.100s[...]'\n", arg );
+		return LIST_OK;
+	}
 	// The server might be weird and have a non-uppercase INBOX. It
 	// may legitimately do so, but we need the canonical spelling.
 	normalize_INBOX( ctx, arg, argl );