afp: Prevent usage of uninitialized variable

More warnings are printed when building thanks to meson port. The code
contains goto command which skips initialization of server_name variable.
Let's move the initialization to another place in order to prevent usage
of uninitialized variable and the following warning:
warning: ‘server_name’ may be used uninitialized in this function

https://bugzilla.gnome.org/show_bug.cgi?id=786149