JavassistHelper: do not rely on isFrozen

When classes are loaded via ClassPool#toClass, they are marked as
frozen, but not their dependnecy classes (even if they were modified
using Javassist, too). Hence we need to be very lenient when it comes
to defining classes after giving ij-legacy a shot.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>