bug#24784: 26.0.50; JSON strings with utf-16 escape codes

[Helmut Eller]

json-read-from-string doesn't parse strings correctly if the the \u
syntax is used to write UTF-16 surrogates:

 (equal (json-read-from-string "\"\\uD834\\uDD1E\"") "\"\U0001D11E\"")
 => nil

The correct result t.  To quote RFC 7159[*]:

   To escape an extended character that is not in the Basic Multilingual
   Plane, the character is represented as a 12-character sequence,
   encoding the UTF-16 surrogate pair.  So, for example, a string
   containing only the G clef character (U+1D11E) may be represented as
   "\uD834\uDD1E".

[*] https://tools.ietf.org/html/rfc7159#section-7


In GNU Emacs 26.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.5)
 of 2016-10-24 built on caladan
Repository revision: 26ccd19269c040ad5960a7567aa5fc88f142c709
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.5 (jessie)

Configured using:
 'configure --with-xpm=no --with-jpeg=no --with-gif=no --with-tiff=no'

Configured features:
PNG SOUND DBUS GSETTINGS NOTIFY GNUTLS LIBXML2 FREETYPE XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: C.UTF-8
  locale-coding-system: utf-8-unix