BUG #13762: server will crash after superuser alter function and set client_encoding

The following bug has been logged on the website:

Bug reference:      13762
Logged by:          funnyxj
Email address:      funnyxj@qq.com
PostgreSQL version: 9.4.5
Operating system:   linux
Description:

When superuser execute the following statements, the server will crash, it
may be a bug.
statements:

1. alter function koi8r_to_utf8(integer, integer, cstring, internal,
integer) set work_mem = default ;
2. set client_encoding = 'koi8';
3. select 1;

then the error log is:
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>