resize with negative values

magick happily accepts `-resize -20` and uses the same dimensions when converting. Not sure what should happen, but anyways, not a panic. ``` $ RUST_BACKTRACE=1 wm-convert m.png -resize -20 x.webp thread 'main' (1243462) panicked at src/operations/resize.rs:360:5: assertion failed: target_width.is_some() || target_height.is_some() stack backtrace: 0: __rustc::rust_begin_unwind 1: core::panicking::panic_fmt 2: core::panicking::panic 3: wondermagick::operations::resize::compute_dimensions 4: wondermagick::operations::Operation::execute 5: wondermagick::plan::ExecutionPlan::execute 6: wm_convert::main note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace. Aborted (core dumped) ```