Log Message:
Update to 1.91. From the changelog:

* Several security bug fixes with malicious GIFs.
* Kill a use-after-free error reported by @junxzm1990.
* Add SIMD support for resizing. When enabled this improves resize
  performance enormously for complex resize methods.
* Add thread support for resizing. `-j[NTHREADS]` tells gifsicle to use
  up to NTHREADS threads to resize an input image. There are several
  caveats---multiple threads can be only used on unoptimized images.
  Thanks to Github user @wilkesybear.
* Quashed several crashes and undefined behaviors. Thanks to Github users
  including @pornel, @strazzere, and @b0b0505.
* Minor bug fixes.