BUG #14917: process hang on create index

The following bug has been logged on the website:

Bug reference:      14917
Logged by:          Boris Sagadin
Email address:      sagadin@gmail.com
PostgreSQL version: 9.5.10
Operating system:   Ubuntu 16.04, CentOS 7
Description:

This happens just on one customer table, UTF-8 is used. Can recreate and
provide more details.

CREATE INDEX on this table never ends. Data can be inserted into table.
Table can be dumped and recreated, but creating index always hangs the
process.

perf top -u postgres on CenOS 7.4, on Ubuntu is very similar.

97.24%  libc-2.17.so      [.] get_next_seq
   0.94%  libc-2.17.so      [.] __strcoll_l
   0.50%  postgres          [.] pg_detoast_datum_packed
   0.21%  libc-2.17.so      [.] __memcpy_ssse3_back