Fix megolm decryption of UTF-8
Repeat the fix from b10f90d for megolm messages. It turns out that the 'length' argument to 'Pointer_stringify' doesn't work if the input includes characters >= 128. Rather than try to figure out which methods can return UTF-8, and which always return plain ascii, replace all uses of Pointer_stringify with a 'length' argument with the version that expects a NULL-terminated input, and extend the buffer by a byte to allow space for a null-terminator. In the case of decrypt, we need to add the null ourself. Fixes https://github.com/vector-im/vector-web/issues/2078.