bug#16686: 23.1; hexl-insert-hex-char shortens file by 7 bytes

[Jacob Burckhardt <jburckhardt@pdvcorp.com>]

hexl-insert-hex-char is supposed to change a character meaning the file
size should remain the same.  But it actually deletes 7 bytes from the
file.  Here is how to reproduce it:

* In the shell, I ran: env printf
'\xee\x00\x4a\x7b\x30\x90\x00\x40\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0
0\x00\x00\x00\x00\x00' >! /tmp/testfile

* In the shell, I ran "env ls -l /tmp/testfile".  It output:

-rw-r--r-- 1 jburckhardt jburckhardt 23 Feb  7 17:27 /tmp/testfile

* emacs -q /tmp/testfile &

* M-x hexl-mode <return>

* M-x hexl-insert-hex-char <return>

* ff <return>

* C-x C-s

* Type 'y' to acknowledge the warning about loss of undo info.

* In the shell, I ran "env ls -l /tmp/testfile".  It output:

-rw-r--r-- 1 jburckhardt jburckhardt 16 Feb  7 17:29 /tmp/testfile

Notice that the first ls command said 23 bytes, but the last ls command
said 16 bytes.  So it lost 7 bytes.  The second ls command should have
also said 23 bytes.

This happened on GNU Emacs 23.1.1 on a machine whose "uname -a" says:

Linux bb003-centos-02 2.6.32-358.14.1.el6.x86_64 #1 SMP Tue Jul 16
23:51:20 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

-Jacob