crash on malformed png files

[Fil]

Steps To Reproduce The Error

❯ echo "hello" > file.png
❯ imageoptim --imagealpha  file.png
i Running ImageAlpha...
! Command failed with exit code 2 (ENOENT): /Applications/ImageAlpha.app/Contents/MacOS/pngquant --ext=.png --force --skip-if-larger --speed=1 --quality=65-80 256 -- /var/folders/57/f83bftsx79b9ylc2gzbdmdfw0000gp/T/imageoptim-cli/file.png

! Please raise an issue at https://github.com/JamieMason/ImageOptim-CLI/issues

    Error: Command failed with exit code 2 (ENOENT): /Applications/ImageAlpha.app/Contents/MacOS/pngquant --ext=.png --force --skip-if-larger --speed=1 --quality=65-80 256 -- /var/folders/57/f83bftsx79b9ylc2gzbdmdfw0000gp/T/imageoptim-cli/file.png
        at makeError (/usr/local/Cellar/imageoptim-cli/3.0.2/libexec/lib/node_modules/imageoptim-cli/dist/imageoptim:6571:11)
        at handlePromise (/usr/local/Cellar/imageoptim-cli/3.0.2/libexec/lib/node_modules/imageoptim-cli/dist/imageoptim:6368:26)
        at process._tickCallback (internal/process/next_tick.js:68:7)

Expected Behaviour

ignore or report malformed png files

Actual Behaviour

crash

Version Numbers or N/A

• macOS: 10.15.7
• /Applications/ImageOptim.app: 1.8.8 (brew)
• /Applications/ImageAlpha.app: 1.5.1 (brew)
• /Applications/JPEGmini.app: N/A

Help Needed

would the test at

ImageOptim-CLI/src/is-supported.ts

Line 4 in e65bb4a

supportedFileTypes.indexOf(extname(filePath)) !== -1;

be complemented by a magic number test on the actual file's contents?