bug#13262: 24.3.50; crash when multibyte directory name completion.

[yfb02119]

I encounter Emacs crash when I try to complete directory with multibyte
name.  I attach `bt full` output.

This is my reproduce steps.
1. mkdir /tmp/aあb
2. In Emacs, M-x find-file, input '/tmp/a', then hit TAB.
   mini-buffer shows "Find file: ....//tmp/aあb/".
3. Hit return key.

GNU Emacs 24.3.50.1 (i386-mingw-nt6.1.7601)
Bzr revision: 111247 jay.p.belanger@gmail.com-20121216020730-iik3hbvw4uxgvugz
Windowing system distributor `Microsoft Corp.', version 6.1.7601

Important settings:
  value of $LANG: ja_JP.SJIS
  locale-coding-system: cp932
  default enable-multibyte-characters: t

bt_full
Description: Binary data