pytalloc: fixed py_talloc_steal()

pytalloc: fixed py_talloc_steal()

py_talloc_steal() was implemented as a macro which evaluated it's 2nd
argument twice. It was often called via a macro with a 2nd argument
that was a function call, for example an allocation in
py_talloc_new(). This meant it allocated memory twice, and leaked one
of them.

This re-implements py_talloc_steal() as a function, so that it only
does the allocation once.