BUG #9068: invalid YY_START on Unicode identifier escape syntax

The following bug has been logged on the website:

Bug reference: 9068
Logged by: Manuel Gómez
Email address: targen(at)gmail(dot)com
PostgreSQL version: 9.3.2
Operating system: Linux
Description:

Long story short:

manuel(at)debian $ psql
psql (9.3.2)
Type "help" for help.

manuel=# U&""""
invalid YY_START
manuel(at)debian $

I was toying around with the Unicode identifier escape syntax and stumbled
upon this. It seems to work correctly if the identifier isn’t the last
token in the input:

$ psql
psql (9.3.2)
Type "help" for help.

manuel=# select 42 as U&"""";
"
----
42
(1 row)

manuel=# select 42 as U&""""+
manuel-#

(psql waits for the rest of the command)

A very minor annoyance, really, but still worth reporting since it kills the
client.

Observed identical behavior on 9.3.2 in 64-bit Debian 7.2 and 9.3beta1 on
32-bit Ubuntu natty.