BlockNumber initialized to InvalidBuffer?

Hackers,

I stumbled across an initialization of a BlockNumber with InvalidBuffer,
which seems strange to me, as the values for "invalid" of the two types
are different, see attached patch.

In case the 'stack' argument passed to that function is not NULL, the
variable in question gets overridden immediately, in which case it
certainly doesn't matter. I don't know nor did I check whether or not it
can ever be NULL. So this might not be a real issue at all.

Regards

Markus Wanner