bug#3668: 23.1.50; next-line and previous-line don't keep the column in text-scale-mode

When text-scale-mode is active the functions C-n (next-line) and C-p
(previous-line) don't stay at the same column when they are executed the
first time after changing the column. Difficult to explain but here's a
simple way to reproduce:

 1. Start Emacs:

        emacs -Q

 2. Increase or decrease the font size with C-x C-- or C-x C-+
    (text-scale-adjust) so that it's different from the default size.

 3. Type this text in the *scratch* buffer at column 0:

        123
        456

 4. Move the point at the beginning of line "123".

 5. Use C-f to move the point between numbers 2 and 3.

 6. Use C-n to move to the next-line.

 7. The point should be between 5 and 6 but it is between 4 and 5.

 8. Fix the bug. :-)



in GNU Emacs 23.1.50.7 (i686-pc-linux-gnu, GTK+ Version 2.12.12)
 of 2009-06-24 on mithlond
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--prefix=/home/dtw/local''