Error check in TKey.cxx

In TKey.cxx. line 776

http://root.cern.ch/root/html/src/TKey.cxx.html#776

One could potentially make arithmetic with NULL pointer, which is not good.
I propose to check pobj before performing arithmetic.

if (!pobj)

{ Error("ReadObj", "Cannot create new object of class %s", fClassName.Data()); return 0; }

TObject* tobj = (TObject*)(pobj+baseOffset);