Segmentation fault if changing input_filename/output_filename attribute before starting GroupCall

[L3Nya]

Segfault happens if you change input_filename or output_filename attribute of GroupCall before starting it.

Steps to reproduce:

1. create GroupCall instance.
   gc = pytgcalls.GroupCall(client, "", "")
2. set its input_filename attribute to anything before starting
   gc.input_filename = "input.raw"
3. catch lovely segfault.