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@gmail.com
PostgreSQL version: 9.3.2
Operating system:   Linux
Description:

Long story short:

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

    manuel=# U&""""
    invalid YY_START
    manuel@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.