Fix issue11379 divide by zero in JPEGReaderPlugin

Fix issue https://pharo.fogbugz.com/f/cases/11379/Fix-divide-by-zero-protection-in-JPEGReaderPlugin

There was 2 possible divisions by zero, // dx and // dy, but only the case when both dx and dy are zero were protected.

This fix has been applied in Squeak VMMaker and VMMaker.oscog branches already.