BUG #14468: One byte buffer overlow in quote_literal_cstr()

The following bug has been logged on the website:

Bug reference: 14468
Logged by: Heikki Linnakangas
Email address: hlinnaka(at)iki(dot)fi
PostgreSQL version: 9.6.1
Operating system: All
Description:

postgres=# select format('%L', E'\\');
WARNING: detected write past chunk end in ExprContext 0x55c65ff98fa8
format
--------
E'\\'
(1 row)

This was originally reported against Greenplum, at
https://github.com/greenplum-db/gpdb/issues/1301. The code there isn't quite
the same, but it turned out to be a shared bug.

Looking at quote_literal_cstr(), it simply doesn't take into account the
space needed for the NULL terminator, when it allocates the buffer. The fix
is a straightforward + 1 to the allocation. I'll go and do that shortly, and
backpatch.