If a null Object reference is box()'d and unboxable!(SubClass)(boxedNullObj) is
attempted, std.box.Box.unboxable tries to offset from null.

: import std.boxer, std.stdio;
:
: class Hmm {}
:
: void main ()
: {
:       // need this to get TypeInfo for Hmm, else I get linker error
:       if (typeid(Hmm) is null && false)  writefln("never");
:
:       Object nullObj;
:       Box nullBox = box(nullObj);
:
:       bool isUnBoxable = unboxable!(Hmm)(nullBox);
:
:       writefln("isUnBoxable: %s", isUnBoxable);
: }

: $ ./boxbug
: Box.unboxable(Hmm) begin...
:         this.type = Object  this.data = 0xbffff2c0(4)[00,00,00,00,]
:         both are classes, casting this.data to an Object reference...
:         o is null: true
: Uncaught:
: SegmentationFault: unmapped address 0x00000000

After fixing this, I discovered another bug: 'template unbox (T : Object)' will always return null if the boxed thing is a null-Object-reference regardless if it should be unboxable as the requested type, causing 'template unboxTest (T)' used in the unittest to fail.

(On a side note, bug
http://www.digitalmars.com/drn-bin/wwwnews?digitalmars.D.bugs/4172 is preventing
'bit fails (void delegate()func)' in the unittest from working, so you have to
do all the fails cases one by one.)

I've attached my patch.