bug#25545: 26.0.50; long values in *Help* are truncated

[Mark Oteiza]

>From -nw -Q in a terminal of width 80:

1. Insert into *scratch*:

  (defcustom my-long-defcustom-name-for-reasons-foo 
'a-long-variable-name-because
    "A docstring"
    :type 'variable)

2. Evaluate it
3. C-h v RET my-long-defcustom-name ... RET

The *Help* buffer's first two lines are

my-long-decustom-name-for-reasons-foos value is
a-long-variable-name-becaus

missing the ultimate letter "e".  I had noticed this in some long string
values where the ultimate double quote was missing.

In GNU Emacs 26.0.50.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2017-01-25 built on logos
Repository revision: ff9ccf6883113ef6d30a352f3ee6dfaa090f0e88