aptitude/dpkg fails at "Preparing to unpack"

[Bill Brelsford <wbrels@k2di.net>]

Recent security updates on jessie (i486) have failed:

  # aptitude update
  ...
  # aptitude -DPR safe-upgrade
  The following packages will be upgraded: 
    libgdk-pixbuf2.0-0  libgdk-pixbuf2.0-common  
  ...
  Reading changelogs... Done               
  (Reading database ... 72512 files and directories currently installed.)
  Preparing to unpack .../libgdk-pixbuf2.0-0_2.31.1-2+deb8u2_i386.deb ...
  E: Sub-process /usr/bin/dpkg exited unexpectedly
  Failed to perform requested operation on package.  Trying to recover:

Installing or removing specific packages also fails.  apt-get works
fine, but subsequent aptitude runs fail the same way.

My other jessie system doesn't have this problem, so I assume
something is corrupt.  But I can't find a way to repair it.
Suggestions?  (Aptitude version is 0.6.11-1+b1, dpkg 1.17.25.)