bug#38990: Update of tdlib to 1.5.4 breaks emacs-telega

[Diego Nicola Barbato <dnbarbato@posteo.de>]

Hi Guix,

emacs-telega doesn't work with version 1.5.4 of tdlib (last known good
commit: df23842; first known bad commit: 70848cd).

After starting telega (M-x telega RET) the *Telega Root* buffer looks
like this:

--8<---------------cut here---------------start------------->8---
Status: Ready                 Fetching chats..
[2:All               ]  [0:Secrets           ]  [0:Bots              ]  
[0:Groups            ]  [2:Private           ]  [0:Channels          ]  

--------------------------------(all)---------------------------------
{Alice                       }  Last message from Alice       05.01.20
{Bob                         }  Last message from Bob         04.01.20
--8<---------------cut here---------------end--------------->8---

The dot animation after "Fetching chats" keeps looping and the following
error message is displayed (2 times, according to the *Messages* buffer)
in the minibuffer:

--8<---------------cut here---------------start------------->8---
error in process filter: Wrong type argument: stringp, nil
--8<---------------cut here---------------end--------------->8---

Deleting the ~/.telega directory and logging in again results in the
same error.

The state in ~/.telega seems to get corrupted too: Trying to run the
known good telega from before the tdlib update afterwards fails because
the server seems to crash.

Regards,

Diego