Guard against division by zero

[tstarling]

• Add getid3_lib::SafeDiv() which does a division with ternary guard. Avoids repeating the denominator expression.
• Rearrange a few formulas to avoid unnecessary divisions. a/(b/c) = a*c/b except that the latter produces an answer when c=0.
• Remediate divisions that may possibly divide by zero, or look suspicious.
• In module.audio.midi.php: remove unused variable
• In module.audio.mp3.php: raise an error for zero channels, since lots of things divide by the channel count.
• In module.audio.voc.php: ignore channels=0, pretend channels=1
• In module.graphic.bmp.php: with ExtractData the RLE modes will divide by zero if width=0. With any compression mode, zero width or height means empty data, so add that as a shortcut.

Motivation: https://phabricator.wikimedia.org/T289189

[JamesHeinrich]

Thanks for this contribution.

One thing I noticed, if you can fix before I merge it:

getid3/module.audio-video.ivf.php:48 it should be timebase_numerator/timebase_denominator but you have denominator twice

[tstarling]

getid3/module.audio-video.ivf.php:48 it should be timebase_numerator/timebase_denominator but you have denominator twice

Done. Sorry I missed this. I added SafeDiv after I was halfway through, so in this line I first added a ternary first and then incorrectly changed it to SafeDiv.