[Patch] Fix RegisterConversionType in unit ConvUtils

As it turns out Delphi (7) disallows registering a TConversionType with a factor of Zero (a fact that is not documented in the offical Delphi docs AFAICS).
For reasons only known to them they chose to not raise an EConversionError, but raise an EZeroDivide exception.
Attached patch implements that.

I would also like to adapt the test program that supposedly was committed in #8c5bc918, but I seem unable to actually find it in the fpc sources.

convutils.registerconversiontype-disallow-zero-factor.diff

(Updated the diff, as it seems that Delphi checks for zero before it checks for duplicates)