bug#25773: 25.2; movement w/ prefix args in rectangle-mark-mode

[Charles A. Roelli]

>From emacs -Q:

- C-u 2 C-p
- C-x SPC
- C-u C-f

Emacs hangs before point can move, and you can exit the hang with enough
C-g's.  C-f works fine without a prefix arg, however.



In GNU Emacs 25.2.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 
10.6.8 (Build 10K549))
 of 2017-02-07 built on gray
Windowing system distributor 'Apple', version 10.3.1038
Configured using:
 'configure --with-modules'

Configured features:
JPEG RSVG NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES