`imageproc::edges::canny` panics when thresholds are both 0.0

    let image = image::GrayImage::new(10, 10);
    let edges = imageproc::edges::canny(&image, 0.0, 0.0);

result

thread 'main' panicked at /home/jyi/.cargo/git/checkouts/imageproc-15e7df7e968bb8fb/ab9e19c/src/edges.rs:135:26:
attempt to subtract with overflow
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a4cb3c831823d9baa56c3d90514b75b2660116fa/library/std/src/panicking.rs:681:5
   1: core::panicking::panic_fmt
             at /rustc/a4cb3c831823d9baa56c3d90514b75b2660116fa/library/core/src/panicking.rs:75:14
   2: core::panicking::panic_const::panic_const_sub_overflow
             at /rustc/a4cb3c831823d9baa56c3d90514b75b2660116fa/library/core/src/panicking.rs:178:21
   3: imageproc::edges::hysteresis
   4: imageproc::edges::canny
             at /home/jyi/.cargo/git/checkouts/imageproc-15e7df7e968bb8fb/ab9e19c/src/edges.rs:52:5
   5: vic::baka
             at ./src/main.rs:1021:17
   6: vic::main
             at ./src/main.rs:1030:5
   7: core::ops::function::FnOnce::call_once
             at /home/jyi/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Start in src/edges.rs at canny and the hysteresis code around line 135, then run the supplied 10x10 GrayImage example with both thresholds set to 0.0. Trace how the threshold values reach the subtraction that overflows and confirm the intended zero-threshold behavior with maintainers; done means the reproduction no longer panics and the behavior is covered by an appropriate regression test.