oxipng corrupts an all transparent png file

[bvinc]

$ wget -qO space_16x16.png https://image.ibb.co/n9SLyv/space_16x16.png

$ oxipng space_16x16.png 
Processing: space_16x16.png
    16x16 pixels, PNG format
    4x8 bits/pixel, RGBA
    IDAT size = 18 bytes
    File size = 133 bytes
Reducing image to 2x8 bits/pixel, Grayscale + Alpha
Trying: 8 combinations
    IDAT size = 18 bytes (0 bytes decrease)
    file size = 129 bytes (4 bytes = 3.01% decrease)
Output: space_16x16.png

$ oxipng space_16x16.png 
Processing: space_16x16.png
thread 'main' panicked at 'index 1056 out of range for slice of length 1040', /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/slice.rs:578
note: Run with `RUST_BACKTRACE=1` for a backtrace.

$ RUST_BACKTRACE=1 oxipng space_16x16.png 
Processing: space_16x16.png
thread 'main' panicked at 'index 1056 out of range for slice of length 1040', /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/slice.rs:578
stack backtrace:
   1:     0x555c7f1be15c - std::sys::imp::backtrace::tracing::imp::write::h1d59ca58eb86a1e2
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:42
   2:     0x555c7f1c1e6e - std::panicking::default_hook::{{closure}}::hc8550e2dc230bf9b
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:351
   3:     0x555c7f1c1a74 - std::panicking::default_hook::he85ae9e5c9867198
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:367
   4:     0x555c7f1c230b - std::panicking::rust_panic_with_hook::h319375f6b98710b0
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:555
   5:     0x555c7f1c21a4 - std::panicking::begin_panic::hbfd2ece4b73bdca6
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:517
   6:     0x555c7f1c20c9 - std::panicking::begin_panic_fmt::h8409f0145843dd1f
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:501
   7:     0x555c7f1c2057 - rust_begin_unwind
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:477
   8:     0x555c7f1ee2bd - core::panicking::panic_fmt::hfaaf47fca562bd04
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/panicking.rs:69
   9:     0x555c7f1ee3b9 - core::slice::slice_index_len_fail::h1479b49feeda4eb6
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/slice.rs:578
  10:     0x555c7f0c8df8 - <oxipng::png::ScanLines<'a> as core::iter::iterator::Iterator>::next::h0c866b7002f03a5d
  11:     0x555c7f0cab17 - oxipng::png::PngData::unfilter_image::hd29ee8bdcb3c42ba
  12:     0x555c7f0ca124 - oxipng::png::PngData::from_slice::hcb8961728318e794
  13:     0x555c7f0c9018 - oxipng::png::PngData::new::h3d5f02c762088b4b
  14:     0x555c7f0d93ad - oxipng::optimize::h1ce3773fa9751aac
  15:     0x555c7f0b0794 - oxipng::handle_optimization::hf840bd1282ba2ff4
  16:     0x555c7f0ad099 - oxipng::main::h448741f3952fecea
  17:     0x555c7f1c919a - __rust_maybe_catch_panic
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libpanic_unwind/lib.rs:98
  18:     0x555c7f1c2a76 - std::rt::lang_start::h0637c2e100ff36fc
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:436
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panic.rs:361
                        at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/rt.rs:57
  19:     0x7f87ebfb53f0 - __libc_start_main
  20:     0x555c7f0a3f19 - _start
  21:                0x0 - <unknown>

[bvinc]

This is a good case to add a verification check at the end that verifies that the output png file can be read and that all the pixels are the same.