Fix bad cast of virtual class to base class via reinterpret_cast

Clang 3.4 found it:
qv4managed_p.h:202:9: error: 'reinterpret_cast' to class 'QQmlDelegateModelGroupChangeArray *' from its base at non-zero offset 'QV4::Managed *' behaves differently from 'static_cast' [-Werror,--Wreinterpret-base-class]

QV4::Managed and QV4::Object are non-virtual classes (they have no
virtual table). I'm not sure if they are (C++11) standard layout, but
they seem to fit the bill. However, QQmlDelegateModelGroupChangeArray
has virtual functions, so the QV4::Managed sub-object in that class does
not start at offset zero. That means reinterpret_cast'ing the base to
the derived class is *wrong*, even if we're just calling a static
function.

In any case, we're static_cast'ing in the next line anyway, so this
can't hurt.

Change-Id: Icc796f7ecf8f41f859ea5fc877f5db5c87799964
Reviewed-by: Lars Knoll <lars.knoll@digia.com>