[Exim] Small bug in 3.20 authenticated_sender assignment

There is a bug in auth_xtextdecode: It does not null-terminate the string.
Yes, the contents may be binary, but they are assigned to the char*
authenticated_sender, which is used in expansions etc.

I don't know if this is already fixed in 3.21: The file should be called
xtextdecode.c, not xtestdecode.c, in both src/auths and scripts/MakeLinks.

Michael