Check for null return from com.kenai.jffi.InvokeDynamicSupport.getFastNumericInvoker()
Fix #494 by moving the sentinel nulling until after freeing the native memory allocation.
Add -Xffi.compile.reify=true|false switch to reify compiled FFI invokers.  It is off by default, because it generates one class per native method, instead of one class per native method signature, and hence will consume more permgen.