bug#23298: 25.0.92; electric-pair-delete-pair doesn't check if parens are balanced

[Mohammed Sadik <sadiq@sadiqpk.org>]

When pressing Backspace key, with electric-pair-mode enabled,
it unconditionally deletes the following char, if its the matching pair.
Ideally, it may better check if it breaks the equilibrium of
braces/quotes.

Eg: code: (char-before (point))
                             |_ point location
If my point (cursor) is just before the last paren (the cursor is
blinking on the last paren), and I press backspace until the beginning
of "point"[0], and then one char back[1], it just deletes the bracket,
without checking if the parens are balanced.

code:
        [0] (char-before ()
                         |_point location
        [1] (char-before
                        |_ point location

This could be the case of quotes, braces, etc.

In GNU Emacs 25.0.92.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.9)
 of 2016-04-13 built on fedora.localdomain
Windowing system distributor 'Fedora Project', version 11.0.11800000
Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS
LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LC_MONETARY: en_IN.UTF-8
  value of $LC_NUMERIC: en_IN.UTF-8
  value of $LC_TIME: en_IN.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  electric-pair-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Quit
Electric-Pair mode enabled
41 (#o51, #x29, ?\))
Making completion list...

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec epg epg-config gnus-util mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util help-fns help-mode easymenu cl-loaddefs pcase
cl-lib mail-prsvr mail-utils cus-start cus-load elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel x-win term/common-win x-dnd tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 93411 4512)
 (symbols 48 20513 0)
 (miscs 40 79 146)
 (strings 32 15817 4612)
 (string-bytes 1 434978)
 (vectors 16 11738)
 (vector-slots 8 421568 5007)
 (floats 8 166 249)
 (intervals 56 218 10)
 (buffers 976 12)
 (heap 1024 46579 1042))