bug#24672: 26.0.50; viper-mode + c-mode: "ce" fails in cpp-continued stmt

[Jim Meyering]

This is with an emacs binary I built from "master" yesterday.

Reproduce with this:

  printf '%s\n' '#define abcde \\' x > k.c; emacs -q -f viper-mode k.c

Then, hit "n" at the viperize prompt and you'll see this
on the first line:

  #define abcde \

Hit "W" to move the cursor to the "a". Then type "ce(" to
change the 5-byte "abcde" to a "(".  I was surprised to see
the "abcde" still there, and an additional backslash appended:

  #define (abcde \                                \

I expected to see this:

  #define ( \

In GNU Emacs 26.0.50.1 (x86_64-unknown-linux-gnu)
 of 2016-10-10 built on hx.meyering.net
 Repository revision: 8b1be820ce02379c9ebd124703e04eee65779662

Configured using:
 'configure --prefix=/p/p/emacs-2016-10-10.08h35 --without-gpm
  --without-x --with-x-toolkit=no --with-png=no --with-jpeg=no
   --with-sound=no'