The removed case (pcHead == script.end()) can never be reached, since
we explicitly error out above if that is the case. It is legacy from
Namecoin's merge-mining, which does not forbid this case earlier.
MakeCoinbaseWithAux() was incorrectly using << to add a CScript onto a
CScript instead of the + operator. Operators corrected and structure
now more closely reflects coinbase script assembly done elsewhere.
